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Strand Boating Guide

Fibreglass / Fiberglass / GRP Boat Guide.

Why GRP?

GRP, the combination of polyester resin and glassfibre, is a remarkably versatile material – strong, durable, weatherproof, waterproof, non-rusting, easily moulded to virtually any shape, highly adhesive to a wide range of materials, and capable of making structures in almost any size. These qualities make it ideal for the construction and repair of boats, and it has been widely used for this purpose for more than forty years. It is used on all kinds of craft from canoes to dinghies to luxury motor yachts and naval minesweepers.

The GRP materials used by commercial boatbuilders are readily available from East Coast Fibreglass Supplies, and you can use them for a variety of projects:

If you’re an ambitious boating enthusiast, you can build your own boat with glassfibre. If you own an old wooden hull, you can renovate it by sheathing it with a glassfibre skin. If you have a metal, timber or GRP hull that’s damaged, you can repair it with GRP materials. If you want to “customise” your boat, you can use the same materials for building all kinds of modifications. And many of these materials are invaluable in fitting-out any type of craft.

What is GRP?

GRP stands for Glassfibre Reinforced Plastic – a plastic resin (usually polyester) reinforced with glassfibre mat or fabric. Polyester resin is a treacly liquid, which when activated by a suitable catalyst (hardener), sets to a hard, rigid plastic. As a liquid, it naturally adopts the shape of its container – hence it can be painted into a mould, and, once cured, will reproduce the shape of the mould. The hardened plastic is brittle so glassfibre materials are added to give considerably increased strength (just as steel rods are used in reinforced concrete). The plastic cures at room temperature (unlike polyethene’s, polypropylenes and other materials which need heat and pressure) workshop (and, for some projects, even in the home), without any need for specialised equipment.

GRP for Boats

The use of GRP revolutionised the boatbuilding industry in the early Fifties. Strand’s parent company, Scott Bader, who first introduced unsaturated polyester resins to Europe in 1946, was at the forefront of the revolution. They produced the first air-drying polyester in 1949 and were involved in the Tod 12, one of the first production series GRP boats, built in 1951. They created the first resins designed specifically for marine use, which were used in the 15m Halmatic Perpetua, the first large GRP boat – and the first to be diesel-powered. Their resins were used in the first GRP hovercraft, Hovermarine’s HM2, in 1967, and in the yacht with which Geoffrey Williams won the 1968 Singlehanded Transatlantic Race (an early example of advanced technology GRP sandwich construction). Scott Bader’s Crystic resins were also used in the 1976 Eclipse, the first to use lightweight Kevlar and PVC foam sandwich construction, and in the 1988 13m Cougar powerboat, a high-performance 95mph craft built with Kevlar and advanced resin hybrid technology. With a record which includes the first thixotropic polyesters, the first polyester fire-retardant coating, the first environmentally safe anti-fouling, the first blister-resistant systems, and many other advances, Scott Bader are easily the UK leaders in polyester technology. Through the Scott Bader network we stock  these high-technology products are readily available for you to use on a variety of DIY marine projects.

Getting Started

One of the advantages of GRP is that you can start a project with very little in the way of tools, equipment, and workshop facilities. Even for quite major projects, the only equipment and workshop facilities. Even for quite major projects, the only equipment required is a few brushes, laminating rollers and plastic buckets! This contrasts sharply with the amount (and cost!) of equipment needed for joinery, metal work and other construction methods.


The following are the GRP materials normally required for most boatbuilding or repair techniques:


A resin which forms the smooth outer surface of the finished laminate and is therefore applied first to the mould. It is thixotropic to prevent it draining from vertical surfaces.

Lay-Up Resin:

General purpose resin used for laying-up glassfibre – approx. one kilo is needed per square metre of glassfibre mat (although this will vary according to the weight and type of glassfibre material). The same resin is ideal for general repair work.


All resins, including Gelcoat, must be catalysed with hardener before use. Uncatalysed resin simply will not harden. Hardener (catalyst) is available as paste or liquid. (Catalyst, especially in liquid form, must be handled with special care – see section on “Precautions”).


Although laminates can be painted, it is simpler to make them self coloured, by adding pigment paste to the resin.


A variety of glassfibre materials (mats, rovings, fabrics, etc.) are available to give a choice of strength-to-weight ratios. The most widely used is Chopped Strand Mat.


Inert filler powders are added to lay-up resin to produce a general purpose body filler/stopper paste for repairs. Glass bubbles are excellent for boat repairs – most other fillers are slightly absorbent and should not be used on boat hulls (or any item likely to be submerged). Fillers are rarely needed for laminating projects.

Release Agents:

Release agents are applied to the mould prior to laminating. They prevent the laminate adhering to the mould surface. 


Used as a thinner, styrene is added to resin to make an initial primer when sheathing wooden structures.

Solution MW:

Reduces surface tackiness in certain resins-required for projects such as boat sheathing, repair patches, etc

Foam Sheet:

Polyurethane or PVC foam sheets are widely used for sandwich construction,and are cut and shaped for use as formers.

Brush Cleaner:

For removing uncured resin from brushes and tools-it can be harmful to the skin and should NOT be used as a hand cleanser. It evaporates quickly and should be kept in covered containers.

Barrier & Cleansing Creams:

To protect or clean the skin – Barrier Cream should be worn in conjunction with gloves when handling resins or glassfibre, and Cleansing Cream (NOT Brush Cleaner) should be used to remove resins, etc, from the skin.

Specialist Resins:

Apart from the standard Gelcoat and lay-up resins, a wide variety of other resins are available for specialist uses such as making chemical-resistant or fire-retardant laminates, etc. For marine projects such as building boat hulls, Resin 491pa (an isophthalic laminating resin) is particularly recommended as a high performance alternative to general purpose lay-up resin.

Tools & Accessories:

Mixing Cups & Buckets:

For mixing resins, pigments, fillers, etc. Most plastic cups or buckets are ideal, except for polystyrene- it is dissolved by the resin!

Brushes and Rollers:

 For applying resin. Use tools specifically made for GRP – conventional decorator’s tools are often use adhesives which are attacked by the resin.

Metal Rollers:

Needed to consolidate the resin/glassfibre laminate.

Catalyst dispenser:

A purpose-made safety dispenser is essential for measuring out liquid catalyst. For small amounts, a measuring syringe can be used.

Plastic Gloves:

You are strongly advised to wear plastic gloves, as well as Barrier Cream, when handling resins or Fiberglass.

Estimating Quantities

To estimate the quantity of materials needed for a specific laminating project, you will need to know the surface area of the mould you will be using. From this, the required quantity of Resin B (Gelcoat) can be calculated – allow 600gm of Gelcoat for every square metre.

The surface area of the mould, in square metres, also gives the amount of fibreglass needed. Multiply the area by the number of layers you intend to lay down (i.e., the required thickness of the finished laminate), and the result is the total glassfibre quantity. Use this quantity to ascertain, from the table below, the amount of lay-up resin required:

For example, 5 layers of 450g matt = 5 x .450 = 2.25kg of matt, multiply by 2.5:1 = 5.6kg of resin, the operator should use between 2 – 2.5:1 resin glass ratio, any more you will be introducing brittleness to the laminate, any less creating a dry and weak laminate.

Resin to Glassfibre Ratios

For an effective laminate, it is essential that each layer of the glassfibre material is thoroughly “wetted-out” with Lay-up Resin. The amount of resin needed to do this varies, depending on the type of glassfibre material:

Chopped Strand Mat, 300gsm

Requires 650gm Resin per square metre

Chopped Strand Mat, 450gsm

Requires 1kg Resin per square metre

Chopped Strand Mat, 600gsm

Requires 1.35kg Resin per square metre

Plain Weave Fabric, 127gsm

Requires 150gm Resin per square metre

Plain Weave Fabric, 200gsm

Requires 250gm Resin per square metre

Satin Weave Fabric, 300gsm

Requires 350gm Resin per square metre

Woven Roving , 290gsm

Requires 300gm Resin per square metre

Woven Roving, 600gsm

Requires 600gm Resin per square metre

Woven Roving, 780gsm

Requires 780gm Resin per square metre

Surface Tissue

Requires 200gm Resin per square metre


Resin Quantity

Black Pigment

Other Pigments

























The pigment quantity can be varied to reach the required depth of colour. The table shows the MAXIMUM amounts which can be used – do not exceed these, as too much pigment will impair the curing process.

Mixing Quantities: Polyester Resins & Catalyst

The proportion of catalyst to resin is normally 1-2ml of liquid catalyst, or 8-16ml of paste catalyst, per 100gm of resin. The catalyst should be stirred thoroughly into the resin.


Liquid Catalyst (ml)

Paste Catalyst (cm)
































With lay-up resin (but NOT gelcoat), the rate of cure can be slowed down by using 1ml liquid catalyst or 8cm paste per 100gm resin – i.e., halve the quantities shown above. This can be useful in very hot weather when the resin might otherwise cure too quickly.


Working Area

When carrying out any GRP project, you should work in an ambient temperature around 20°C, as this ensures that the resin will cure correctly. Resins will not cure adequately below 15°C, and at temperatures above 30°C, they will cure too quickly! If working outdoors, as is often the case with boating projects, you will obviously be at the mercy of the weather. Indoors, you must not use oil heaters or electric fires (see below), so maintain the temperature with convector heaters, “Dimplex” radiators, or similar. You will need trestles to support the mould at a comfortable working height – for small jobs; of course, you can use a normal workbench or trestle table. Cover it with polythene, Polyester Film or brown wrapping paper.


You should have no problems with GRP materials providing you use them with proper care and ensure adequate precautions are taken:

Whether laminating or doing minor repair jobs, always remember that GRP materials produce vapours which are inflammable and which should not be inhaled. Therefore,

ALWAYS work in a well-ventilated area, and NEVER smoke or use naked lights or fires in the work area. When you have finished working make sure spare materials are securely stored – especially potentially hazardous products such as catalyst and brush cleaner.  Wear gloves – do not let any materials contact the skin, and especially the eyes and mouth. Do not swallow any of these materials. Catalyst should be treated with special care: it is inflammable and corrosive – do not let it contact the skin, mouth or eyes. Should it do so, wash from the skin immediately under running water. If it contacts the eyes, flush them under running water for at least 15 minutes and obtain medical attention. Glassfibre, resins and ancillary materials should not be used by children UNLESS CLOSELY SUPERVISED.


Making a Mould

With a few exceptions, the first requirement in nearly all GRP projects is a mould in which to lay up the laminate. Some moulds can be very simple – you can lay up GRP flat panels on no more than a sheet of hardboard faced with self-releasing polyester film. Moulds are male or female, with the laminate applied to the outside of a male and the inside of a female. The finished item has a smooth surface on the side nearest the mould, so a male mould is used when a smooth interior is required (shower trays for example). For boat hulls, a female mould would be used, which gives a laminate with a smooth exterior. In some cases, particularly canoes, it is possible to buy or hire ready-made moulds, but usually you would have to make one yourself. You can use most solid materials for mould making. Wood, hardboard, plaster, clay, steel and concrete can all be used, but the best material is glassfibre itself. The GRP mould is made by laminating over a suitable item. You can take a mould from an existing boat or canoe (although be wary of infringing design copyright!) but usually you will have to make a “plug” (sometimes called a pattern or former) which is an exact mock-up of the finished item.

Making the “Plug”

Almost any material can be used, as long as the plug is accurate, rigid and has a highly finished surface. A small plug can be modelled in clay, or built from ply or fibreboard. Typically, a large plug might have a wooden framework covered with plywood or hardboard, or with clay or plaster reinforced with wire netting or hessian – or any combination of these. The fibreglass mould will faithfully reproduce the plug surface, so it must be totally smooth and unblemished – nails must be hammered well in screws countersunk, with the heads covered with filler, and all dents, joints and seams must be carefully smoothed down then painted with Furane Resin, a coating resin which helps to give a wooden plug a highly glazed surface. If the plug has a deep draught or undercuts, it will be necessary to make the mould in two or more sections; otherwise, the laminate will lock into the undercuts and be impossible to remove.

This is often the case with boat hulls, where the mould may be split along the keel line to allow for “tumble-home” at the stern. These sections will need flanges, so they can be bolted together. To produce the flanges, the plug should have “fins” fitted along the split-lines which separate each section. The fins can be made of any suitable material, such as thin aluminium sheet/ melamine board. Alternatively, the plug itself can be made in sections. When the plug is completed, it must be treated with release agents prior to laminating the glass fibre mould. If the plug is made from porous materials, such as plaster, the surface should be sealed with Release Agent, wax and polish the plug with eight to ten coats of a solid wax such as TR or Honeywax  – buff each coat and leave for an hour to harden thoroughly before the next is applied. When the final layer has hardened, PVA release agent is used to provide a surface coat. The mould can now be laminated over the plug.

The GRP Mould

The mould is produced by the same method used for any other glassfibre lay-up – a layer of gelcoat, followed by successive layers of glassfibre impregnated with Resin. This is explained in detail in the section on Laminating Technique. The only difference is that the mould needs to be thicker than the finished item – it will often need to be twice to three times as thick. This would involve considerable expense on large moulds, but economies can be obtained by the judicious use of strengthening ribs and stiffeners. These will prevent the mould flexing or distorting during use. Add these ribs after the mould has partly cured, or contraction of the surrounding laminate may leave an impression on the mould surface. The ribs are easily made by laminating over a suitable former. A popular material for formers is paper rope, made of paper wound on a flexible wire core. It has a flat side which is laid on the GRP surface in the required position. You then laminate strips of glassfibre over it to produce reinforcing ribs, which give added stiffness with little extra weight. The former itself provides none of the extra stiffness – this results entirely from the box section of the laminated rib.

If the mould is in two or more sections, these will need flanges (preferably about 75mm wide and at least 50% thicker than the rest of the mould) which can be drilled later so the parts can be bolted together. Leave the mould to cure completely (preferably for at least two weeks), unless using specialist tooling resins, before removing from the plug – too early removal can result in distortion. To support a large mould, and make it more rigid, a timber framework can be bonded on. This should be done after the mould has cured, but before it is removed from the plug. After removal, let the mould “breathe” for a few days before filling or sanding any imperfections – there should not be many of these if the plug was properly finished.

Laminating Technique

Preparing the Mould

Unlike moulds made from “self-releasing” materials, such as polyester film, the GRP mould will need to be treated with release agents to ensure the laminate does not bond to the surface. Most release agents are three-part systems: a primary sealer (used to seal moulds made of wood or other absorbent materials, and therefore not needed on a glassfibre mould), a release wax and a glazing agent. Some release agents are one-part systems. After you have finished preparing the surface of your mould wash down with hot soapy water, let dry and apply the first coat of wax, TR 108 is suitable for new moulds, using small circular motions apply, let dry for five minutes and buff off with a clean cotton cloth, and again with another, keep these cloths separate and use the second one after the first each time, this will ensure you are not immediately creating any unnecessary build up.
Repeat this process at least twelve times at least an hour between each application. Do not apply wax within the hour as you will just be dissolving the one you have just put on.
When all the waxes have been applied for insurance it’s best to apply a thin coat of PVA release agent to the first lift, let dry.

After the first lift of your part give another couple of waxes before Gelcoating again, after that wax every lift until you are satisfied that sufficient release is applied, you will be able to tell this by the ease of release of the part, using hard waxes it is common to wax every two – three lifts thereafter.


Preparing Resins

All resins require the addition of catalyst (hardener) to initiate the curing process. Use a safety dispenser to add 10 - 20ml of catalyst per kilo of resin, and pro rata as shown in the table above. Stir thoroughly. The hardening process begins immediately, so only catalyse a working quantity, or your mixing containers will soon be full of solidified resin! If pigments are being used, these should be stirred into the resin before adding the catalyst. Add up to 10% of pigment, depending on the depth of colour required. To maintain consistent colour on a large project, it is often a good idea to pigment ALL the resin, and then decant working quantities to be catalysed as required. Once catalysed, the resin gradually cures, taking on a jelly-like consistency in about 10-20 minutes, before becoming hard in about 30-40 minutes at room temperature (about 20°C). The curing process generates heat (known as “exotherm”) within the resin. Too much catalyst or large volumes of resin increase this heat, so a thick laminate should be built up in stages.

Laminating Small Items

The method for laminating smaller items, such as trays, hatch-covers, etc, is a fairly straight forward. Once the mould has been treated with release agents, prepare some glassfibre by cutting it to the shape and size of the mould (it should overlap the edges by an inch or two all round), then mix up the Gelcoat resin.


Pour the gelcoat resin into a mixing bucket. If the laminate is to be self-coloured (as it usually is!), pigment the Gelcoat if needed with a maximum of one part pigment to ten parts resin. Add 20ml of catalyst per kilo. Stir thoroughly, and then use a brush or polyester roller to cover the mould surface evenly with gelcoat. Wait for at least an hour, until the gelcoat becomes tacky (it will feel slightly sticky but will not actually adhere to the touch). The gelcoat is specially formulated to remain tacky – it facilitates a good bond with the succeeding laminate.


Add catalyst to a suitable quantity of pigmented lay-up resin. Paint the Resin over the gelcoat then cover the mould surface with a piece of glassfibre (cuts darts or tucks if needed to make it fit) and push it gently into the wet resin with a brush. If the fibreglass material is Chopped Strand Mat, use a stippling action – do NOT “paint” to and fro unless well experienced, as this tends to separate the glass fibres. Make sure the glassfibre is thoroughly impregnated with resin (keep the brush well-loaded to add more resin if necessary). Once completely “wetted – out”, use a metal laminating roller to consolidate the layer and force out air bubbles. This also forces resin through the fibres from beneath, which makes for better impregnation.  A further layer of glassfibre can then be added, wetted – out and consolidated, repeating the process. In theory, any number of layers can be built up depending on the thickness and strength required, but in practice, for most small projects, two layers will be adequate. The exposed surface of the chopped strand mat laminate will be rather rough - this will not normally matter, but, if necessary, it can be covered with a layer of surface tissue. Surface tissue is finely textured, lightweight glassfibre material. It has a smooth side and a “hairy” side. Whilst the laminate side is still wet, apply the surface tissue, hairy side down, wetting out with more resin, and stipple down lightly. Surface tissue is not needed (indeed is undesirable) in laminates made with woven glassfibre materials. By now, the resin on the brushes and tools will be starting to solidify. If left, they will shortly become totally hard and unusable. Prevent this by washing them in brush cleaner (acetone) which will help dissolve the resin. They must be wiped dry before re-using. Remember acetone is highly inflammable – do not smoke or use naked lights anywhere near it! It is very volatile, and tends to evaporate, so keep it in a covered container.

Meanwhile, the laminate will begin to cure. It will soon reach “green stage”, when it becomes quite firm, but is not yet hard. At this stage, you can trim off excess material around the edges of the mould with a Stanley knife (once the laminate is fully hardened, it can only be trimmed with a power tool or a hacksaw with a metal-cutting blade). When fully cured, the laminate can be released from the mould. If necessary, wood or plastic (but not metal) wedges can be used to prise out the moulding, but take care not to scratch the surface of either mould or moulding.

Larger Laminating Projects

For large projects such as building a complete hull, the laminating method is essentially the same, although the scale of the job will demand a few slight variations:

If the laminate is to be self-coloured pigment all the resin at the same time to ensure a consistent colour throughout. On a very large project, you may prefer to economise by pigmenting the gelcoat only, but for best results it is wiser to pigment both gelcoat and lay-up resin.


Decant a working quantity of the pigmented gelcoat into a mixing bucket and add 20ml of catalyst per kilo of resin. Stir thoroughly, then use a brush or roller to paint gelcoat over the mould surface. Wait until the gelcoat becomes tacky, which will probably take about an hour. Check any areas where the gelcoat may have drained down a collected (the keel line of a boat hull is a typical example). These concentrations of resin may still be wet. Once you are sure the gelcoat surface is no longer wet, and is consistently tacky, you can proceed to the lay-up.


For laminating a hull, you may prefer to use 491pa high-performance laminating resin, rather than the general-purpose Resin  (both are Lloyd’s approved for Marine use) – in either case, the method used is the same. Add catalyst to a working quantity of the pigmented lay-up resin. Paint the Resin over the gelcoat then lay down a piece of glassfibre. Stipple it into the wet resin with a brush. Remember – do NOT “paint” to and fro, unless well experienced (a resin applicator roller can be used instead of a brush). Make sure the glassfibre is thoroughly impregnated with resin (“wetted-out”), then use a metal laminating roller to consolidate the layer. If the mould is too big to be covered by one piece of glassfibre, lay another piece – this should overlap the first by about 25mm. Wet out, consolidate and repeat, if necessary, until the mould surface is covered. The glassfibre should overlap the edges of the mould, but not by more than 50mm or so (otherwise, the weight of the excess may cause the laminate to pull away slightly from the mould surface). Further layers of glassfibre can then be added, wetted-out and consolidated, depending on the thickness and strength required. A boat hull has to be stronger and therefore thicker, than most laminates. However, you must remember that whilst a thicker laminate will be stronger, it will also be heavier, which can be a critical factor in some applications – choose the right reinforcement to give the necessary strength-to-weight ratio. Also, exotherm generated by the curing resin will produce considerable heat in a thick laminate – avoid this by building the laminate in stages, giving the heat time to dissipate.

A thin laminate can be made stronger or more rigid by adding ribs or box sections, laminated over formers of paper rope, foam or cardboard (see “Formers”). You can also use fibreglass fabrics or woven rovings in conjunction with mat to give the required combination of strength and weight. If you are using a combination of materials, lay the mat first. Do not use two consecutive layers of fabric – always have an intervening layer of mat. The exposed, rough surface of the laminate, if desired, can be covered with a layer of surface tissue, applied whilst the laminate is still wet.

As you are laminating, you will probably have noticed the resin on the brushes and tools already beginning to solidify. Obliviously, you cannot stop to clean them (or the laminate may “go off” whilst you do so) so leave them to soak in Brush Cleaner (Acetone) whilst you continue working with fresh brushes. When the required layers have been built up, leave the laminate to cure. It will soon reach “green stage”, when you can trim off the rough edges with a Stanley Knife (once the laminate has fully cured, it can only be trimmed with a power tool or a hacksaw with a metal-cutting blade). Whilst the laminate is curing, ensure that all brushes and tools are thoroughly cleaned with Brush Cleaner – and ensure they are completely dry before you use them again. When fully cured, the laminate can be released from the mould. If this proves difficult, due to a complex mould or inadequate use of release agents, the moulding can sometimes be sprung out by striking the mould with the flat of the hand – do not use a hammer, as it will probably craze a mould and moulding! A rubber mallet can be used but requires some skill. Wooden or plastic (but not metal) wedges can be used to prise out the moulding but take care not to scratch the surfaces. If PVA Release Agent was used on the mould, pouring luke-warm water between mould and moulding will often facilitate release.


A former is anything which provides shape or form to a GRP laminate. They are often used as the basis for stiffening ribs or box sections. A poplar material for formers is paper rope, a stiff “cable” made of paper wound on a flexible wire core. This is laid on the GRP surface (it can be easily bent to follow the contours) and is laminated over to produce reinforcing ribs, which give added stiffness – this results entirely from the box section of the laminated rib. Wooden strips, metal or plastic tubing and folded cardboard can all be used successfully as formers. A very popular material is polyurethane foam sheet, which can be cut and shaped to any required form. Polystyrene foam cannot be used, as it is dissolved by the resin.

Core Materials

Core materials (usually polyurethane or PVC foam sheets) are used in sandwich construction- basically a laminate consisting of a foam sheet between two or more glassfibre layers. This gives the laminate considerable added rigidity without greatly increased weight. Foam materials are available which can be bent and folded to follow curved surfaces such as storage tanks, without requiring a mould. A variety of materials can be utilised for sandwich construction, marine ply being an obvious example, but of most of them are relatively heavy and can only be used where excessive weight is not a problem.

Epoxy Resins

Epoxy resins are used in some GRP projects, especially for very high performance laminates, in conjunction with reinforcements such as Kevlar. Their use differs somewhat from polyesters, and they also create a number of hazards. Mixing and application methods vary from one epoxy system to another, so it is best to consult the manufactures instructions for each individual product. In general, epoxies are even more adhesive than polyesters, so mould preparation is critical. Polyester gelcoat can be used with epoxy laminating resin, but must be allowed to cure fully (for at least 12 hours) before laminating with the epoxy. Standard glassfibre chopped strand mat, which is normally emulsion-bound, must not be used- you need a special powder bound mat. Fabrics and rovings should also have a special surface treatment suitable for epoxies. The techniques of applying and consolidating the glassfibre layers are the same as those used for laminating with polyester resin. Although pigments can be used, it is more usual to paint an epoxy laminate with two-part polyurethane or epoxy paints. Epoxy resins must be handled with great care – they can cause severe skin irritation. Without gloves and a barrier cream, the skin can become sensitised very quickly.

Liquid Foam Mix

Polyurethane foam mix is a two-part liquid which, after mixing, expands rapidly then sets hard to produce a rigid foam. It can be used to fill cavities such as hollows masts, to which it gives added rigidity. A major use of polyurethane foam mix is for buoyancy applications, since it can support many times its own weight in water. The usual method is to pour the liquid mix into prepared sections of a boat or canoe, allow to foam, then laminate over. To use mix, simply pour equal volumes of both liquids into a mixing container and stir thoroughly. The mixture will start to foam within 25 seconds, and must be poured into the cavity before this time has elapsed. It will reach its maximum rise in about two minutes, when it may expand to between 20 and 25 times its original volume (the amount of expansion varies according to ambient temperature). At this stage, it is highly adhesive, bonding firmly to most materials. It is a good idea to mask off areas to which you do NOT want the foam to adhere. Don’t let it contact the eyes or skin since it is almost impossible to remove. It sets hard within 30 minutes. During the foaming process, harmful fumes are given off – use this material in well-ventilated areas only. Do NOT inhale the fumes and do NOT let the mixture contact the skin or eyes under any circumstances.

Bonding Fittings to GRP

It is often necessary to construct a GRP laminate so that various fittings can be added to it – typical examples are the footrest of a canoe, or the rowlocks of a dinghy, and so on. Such items (and fittings such as “Bighead” fasteners) can often be incorporated directly into the laminate simply by placing them in the appropriate position during laminating. A few small strips of glassfibre can be laminated around the fitting to strengthen the join. Metal plates buried at suitable points within a laminate will give additional strength to GRP items which need to take bolts or screws. A wooden sheet can be used to strengthen the transom of a small boat so it can hold an outboard engine.

Adhesives for Use with GRP

Since polyester resin is highly adhesive, it is the logical choice for bonding most materials to GRP surfaces. Suitable alternatives are:

Sika “Technique” is a heavy duty, polyurethane-based jointing compound. It cures to a flexible rubber which bonds firmly to wood, metal, glass and GRP. It is ideal for such jobs as bonding glass to GRP (e.g., when sealing the windows of a deckhouse) or bonding GRP and metal, as is often required on cars with GRP bodywork. It is not affected by vibration and is totally waterproof. The Crestomer  Range of industrial adhesives which are highly recommended for use with GRP. Most high-strength impact adhesives can be used on GRP laminates. Most other adhesives will be incapable of bonding strongly to GRP and should not be used when maximum adhesion is required.

Repairing Boats with GRP

Polyester resin bonds firmly to a wide range of materials including wood, metal and concrete, and makes strong, durable repair patches which are waterproof, rotproof, non-rusting and resistant to weathering. It can be used to repair almost anything and is especially ideal for mending boats. Minor scratches on GRP surfaces are quite easy to repair simply by filling with gelcoat resin, or with Gelcoat Filler. It may be necessary to gouge out the scratch (preferably with a V-shaped tool) to ensure the resin locks sufficiently. Leave to cure, then smooth down with Rubbing Compound. Small holes, cracks and dents in most GRP, metal or wooden surfaces (other than boats) can be filled with a mixture of lay-up resin and filler powder. You can buy ready-made stopper pastes or bodyfillers, of course, but the advantage of mixing your own is that you can make it to the required consistency (and, in larger quantities, it is more economical). Simply mix filler powder and resin to make a paste, add hardener (10ml per kilo of resin, NOT per kilo of resin/filler mix!), stir, then fill the dent, applying the paste proud of the surface. Leave to harden, then sand down with progressively finer grades of Wet & Dry paper, and paint to match the surrounding surface. The same technique can be used on boat hulls, but you should use Glass Bubbles or Q-Cel rather than conventional filler powder (which tends to absorb moisture, and therefore not recommended for items which are on, or in, water for long periods). You can also make a repair “dough” by mixing lay-up resin and chopped glassfibre strands – this is used in the same way as the bodyfiller but is much stronger and has better gap-filling properties. It can be used on virtually any surface, including boat hulls.

A Fiberglass patch can be used to repair larger holes – in fact, you can use glassfibre to replace entire sections of a damaged hull. There are three ways to replace an extensively damaged area of a hull. One is simply a more elaborate way of patching – cutting and shaping formers (usually of foam material) and laminating over them. Better results are obtained if the damaged section is temporarily “rebuilt” with foam sheet, plaster or any material which can be readily shaped and finished. This is then used as a “plug” from which a mould is taken. A brand new section can then be produced from the mould, the “plug is removed and the new section is laminated into position, with the joints carefully reinforced. Of course, if you are lucky enough to know somebody with an identical boat, you can replace your damaged section by taking a mould from the appropriate section of the second boat!

Epoxy Repair Systems

Epoxy systems are also available for repair work. As different systems vary in their mixing application, it is best to refer to the manufactures instructions for the specific product you intend to use.

Adhesion Treatments

for bending polyester resin to various materials

Polyester resin bonds excellently to wood, plaster, hardboard, stone, slate, brick, plasterboard, chipboard, ferrous metals (eg, cast iron, mild steel), GRP laminates, and polyurethane foam materials. Only a weak bond can be obtained on glass, non-ferrous metals (eg, aluminium), acrylics and acrylates (eg, Perspex). The resin will not bond at all to polythene, latex, most synthetic rubbers (vinyl, silicone, etc), Polyester Film, and Melamine.



Treatment For Maximum Adhesion

Wood, hardboard

Clean. Apply sheathing primer (Lay-up resin thinned with styrene, as described in the section on Sheathing) and leave to Gel before bonding.

Steel & Iron

Clean, score surface and degrease with Acetone.

Non-ferrous metals

Clean and score surface. Apply resin immediately. (Good adhesion will not be possible).

GRP laminates

Clean degrease. Abrade surface and clean again.


Non possible – will not bond.

Plaster, stone, concrete

Ensure surface is clean and dry. Preferably seal with sheathing primer (as wood or hardboard). Fresh concrete or plaster should be left to cure thourghly for at least one month.

Polyurethane Foam

Break surface cells with a rasp or Surform.

Sheet Glass

Clean with dilute hydrochloric acid. (Note: acid must be used with great care).


Not advised. (Is dissolved by polyester resin)


Sheathing is used to provide a wooden surface with a protective glassfibre skin. It can be used on any wooden structure, and once very popular as a means of renovating an old timber hull. It helped to strengthen the hull, seal leaks, improve the appearance, reduce attack by worms or rot, and extend its life. Coating resins have largely superseded this method, but sheathing is still a useful means of giving added protection to cockpits, deckhouses and wooden structures generally.


Glassfibre laminates and resin castings can be sawn drilled, sanded or polished. Most tools intended for metalwork (e g, hacksaws) will be adequate for cutting or trimming laminates and castings, and further shaping can be done with rasps and files – tungsten carbide tools are particularly useful for this. Further finishing can be carried out with progressively finer grades of Wet & Dry paper, used wet with GRP polishing compound being used for the final rubbing down. All of this work can be done by hand or with paper tools. Grit and particles from both glassfibre and the hardened resin can be injurious to eyes and lungs – it is therefore important to wear goggles and purpose-designed breathing masks during finishing, this is absolutely essential when power tools are used.

East Coast Fibreglass Supplies stock an extensive range of Fibreglass materials and Resins suitable for all kinds of moulding and construction work.

This advice and information is given in good faith for guidance only, and is given without warranty, users should determine information given and using their own judgement determine suitability. We cannot accept any liability for any losses. Please read Health and Safety notes on site and general laminating tips. Be extra careful when using Acetone due to the Hazardous nature of the material, observe all hazardous labels that are present on tins