Sheathing boats with Fibreglass
Boat Sheathing with Glassfibre.
Sheathing an existing wooden boat hull with glassfibre provides a protective skin which, if properly applied, can seal leaks, improve the appearance, strengthen the hull (extra reinforcement can be added where desired), withstand knocks or scrapes better than a painted surface, making the hull virtually impervious to attack by worm or rot, and extend the hulls life considerably. Holes can be repaired before sheathing, so the patch cannot lift, annual maintenance of the surface is greatly reduced, and improvements such as non –slip decks be incorporated in the sheathing operation. Some hulls are less suitable than others for sheathing. Hardwoods, because of their low absorbency, are unsuitable as the resin does not penetrate the wood, the laminate tends not to bond sufficiently well to the surface. All softwoods can generally be sheathed with good results. Clinker boats 'move' in two directions and can kick off a hard coating. Epoxy is better than polyester and is therefore more expensive. Done correctly an epoxy sheathed boat will out last you and me, not just last a few years. The secret is, is the boat worth it, and can it be done correctly? Most users opt for Polyester, and this guide is assuming you are using Polyester, if you are using Epoxy you will need to follow a different application guide. There are then two types of Polyester Resins, Isophthalic and Orthophthalic, Isophthalic Polyesters (Crystic 491 pa) is ideal for boatbuilding and sheathing due to their high water resistance and toughness, but again it is more expensive, more commonly used is Orthophthalic. The materials and tools you require for this project are all available in our shop.
Always work in well ventilated area, preferably at a temperature of about 18 - 20 degrees centigrade. Do NOT smoke or use naked lights in the work area. It may be worthwhile to construct a temporary awning or tent. Such a tent should be well ventilated, to avoid fumes from the materials used and to prevent condensation. The area should be equipped with a trestle table or other suitable surface on which to cut the glassfibre material.
Materials and Equipment
General purpose lay – up resin and Flowcoat, Lloyds approved, will be required, together with appropriate quantities of catalyst (hardener), and also a good quality chopped strand matting. A quantity of styrene will also be needed (about 50cc for each kilo of resin on the prime coat). The quantities required will obviously depend on the size of the boat - as an approximate guide; 1 kg of resin will cover about 2 ½ square metres of hull surface as an initial sealing coat. When used for lay – up, 1 kg will impregnate approximately one square metre of 450g/m2 glass mat. The glassfibre reinforcement advised is chopped strand mat, which is available by the metre in rolls 92 cm wide, in a variety of weights. One layer of mat will probably be adequate for small boats up to 3 mt, for additional strength, or for large boats (between 3 and 9 mt) use at least two layers of 450g mat. Some users advocate glassfibre cloth instead of mat, as it requires less finishing and gives good flexural strength, however, it can be prone to separating from the hull, unless very well keyed...
The hull surface must be completely accessible. A small dinghy can be simply inverted but a bigger boat you will probably have to roll the boat on each side alternatively, completing the sheathing in two halves. It may be necessary to use trestles or scaffolding to gain easy access. Prepare the hull surface, by stripping off all old paint, and ensure the timber is clean and dry. Stripping is best done mechanically, using a belt sander or rotating disc. Do not use a chemical paint stripper or a blow lamp-either may leave deposits on the wood, affecting adhesion of the resin. Fill cracks with catalysed resin putty, leave to harden then rub down flush with the hull. Make sure the entire hull is clean and dry.
All resins must be catalysed before use. Lay –up requires 1% catalyst minimum dependant on ambient temperature (10cc catalyst per kg). Use a purpose designed dispenser for measuring the catalyst, which must be thoroughly stirred into the resin. If catalyst is accidentally split on the skin wash it off at once under running water. If splashed in the eyes, flush under running water for at least fifteen minutes and seek medical attention Up to 10% pigment can be added to the resin if needed, and should be thoroughly stirred in. To ensure coverage, it is advisable to pigment enough resin to cover the entire hull. Working quantities can then be decanted and catalysed as required. Normally only the surface coat needs to be pigmented, but, if certain colours are used (reds, yellows and dark blues) it is advisable to pigment the lay-up resin as well. If pigmented resin is left for more than 12 hours; it should be thoroughly stirred before use.
Make sure the surface is dry then prime the surface with Resin, thinned with styrene monomer (1 part styrene to 20 parts resin) and catalysed with 2% catalyst. The wood should be completely impregnated. Leave to cure to tacky condition, then cover with a coat of un-thinned catalysed Resin. Immediately apply reinforcing glass tape, or strips of chopped strand mat, to areas where additional strength may be required e.g. keel, bows, or transom. With a brush, stipple Resin into the reinforcing strips, working the resin/glass well into angles and corners and use a metal roller to release air bubbles. Cover the hull with the pre- cut pieces of mat. Overlap each piece by about 50mm minimum, work and roll matting and feather the join so it is not noticeable. It’s best laying pieces the entire length of the hull (where possible) this will reduce the number of joints and give a better finish. Use the brush with a ‘stippling’ action to push the mat into the resin – do not brush backwards and forwards as this separates the fibres, apply the resin with a polyester roller if you wish. Ensure that the matt is thoroughly wetted through and add more resin if necessary, but be careful not to ‘over wet’, keep the resin:glass ratio to 2.5:1 maximum, work out any wrinkles or air bubbles, especially on the corners of chines, keel, bows and transom. The process is sometimes easier if the matt is stapled in position. Use the same method to build up further layers of mat as required, do not leave more than 24 hours between laminations as the adhesion will be greatly reduced, if left more than 24 hours, rub over with course glass paper and wipe lightly with Acetone to sensitize the surface, ensure the joins in each layer are overlapped by the succeeding layer by at least 50mm. Before the last layer is set, cover it with surface tissue.
For the surface coat you will need Flowcoat, this is best conditioned at room temperature before use to achieve correct viscosity, catalyse with 20cc of hardener per kilo of mix and brush the mix carefully over the hull, allow to harden. If you are working outside make sure the weather is correct,i.e warm and dry with little or no wind. Do not Flowcoat late in the day as the temperature drops, thus allowing the curing cycle to be extended, you are far better applying midday when the conditions are just right. It is particularly important to apply the final coats in warm, dry and well ventilated environment.
The Flowcoat should set with a smooth surface finish, but this can be considerably improved by polishing, using a special polishing compound and an electrically powered compounding head. If necessary, cut the surface back with 240 grade wet and dry paper, working then up to 800 grit, then if need be 1200 grit, after this you can then polish. When the resin is fully cured (allow 10 – 14 days at 20 degrees centigrade), the hull may now be launched. The cure may take several weeks at low temperatures, but this is not recommended – the curing temperature should not be less than 15 degrees centigrade. A white bloom appearing on the surface means that the resin was not fully cured. This will not harm the sheath. When convenient the boat should be dried out and wiped over slightly with acetone, then a wax polish applied. If this does not cure the bloom, cut back the surface and re-polish.
Maintenance of the Sheathed Hull
In service the sheath needs very little attention. If it is damaged it should be repaired as soon as possible, using mat and resin and overlapping the good by at least 20cm, although impervious to attack by rot and worms the surface will be subject to fungal and weed growth, so a coat of anti-fouling is desirable.
Please note before undertaken any major repair work you should seek your own professional advisor. 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.