Blistering of below the waterline gelcoat surfaces, along with the resulting interlaminate failure, have become an increasingly widespread problem. If left unattended, blistering can lead to serious structural damage. Repairing the blister with epoxy and then coating the hull with an epoxy barrier coat is presently the most dependable solution and prevention for this problem.
Solution 5a - Prevention & Repair of gelcoat blisters
1. Prepare the hull for drying by opening all blisters and abrading the bottom with 50-grit sandpaper. For extensive blistering, grinding or sandblasting the gelcoat surface down to the fibreglass laminate may be necessary. The entire surface below the waterline should be sanded to a dull finish, with all bottom paint removed.
2. After the blisters have been opened and the hull sanded, allow the hull's moisture content to stabilise. Two weeks at 50% or less relative humidity is ideal. If more humid conditions prevail, a hot air gun or heat lamps may speed the job. A heavy plastic skirt hung around the boat will keep rain from the work area should conditions deteriorate. A dehumidifier or fan placed under the skirt may speed drying.
3. Salt deposits or other contaminates may be left on the surface as moisture evaporates from the laminate. Wash the surface with fresh water and immediately dry with clean paper towels. When dry, sand the surface once again to provide a clean bonding surface.
4. Apply a single coat of WEST SYSTEM epoxy to the surface with a foam roller. Be sure the inner surface of blisters and pits are well coated.
5. Within 6-8 hours of applying the first coat, trowel a non-sagging fairing mixture of epoxy and 411 Microsphere Blend into the blister cavities.
6. After 24 hours, sand the entire surface to a dull, smooth finish with 80-grit sandpaper. Repeat steps 5 and 6 as necessary.
7. Follow instructions 3 and 4 under "Solution 5b Barrier coating - new or repaired hulls."
Solution 5b - Barrier Coating- new or repaired hulls
1. If the bottom has not yet been painted, wipe the entire surface with a cloth dampened with a wax and silicone remover or solvent to remove remaining mould release agents or waxes.
If the bottom has been painted, the paint must be completely removed with chemical strippers and / or sanding prior to coating.
2. Abrade the gelcoat surface below the waterline with 80-grit sandpaper. When completed, there should be no glossy areas. Mask freeboard at the waterline with electricians tape.
3. Apply a minimum of three coats of WEST SYSTEM epoxy to the surface with a foam roller. For optimum moisture exclusion, we recommend five or six coats. Note: For best results, apply at above 16°C 40-60% relative humidity.
The first coat should be applied with no additives. Each successive coat should include 10% 422 Barrier Coat additive (an aluminium-based additive which increases the epoxy's resistance to moisture permeability) blended with the resin/hardener.
All coats may be applied in one day. Apply the next coat as soon as the first will support its weight. If epoxy is allowed to cure overnight wash with water and a 3M Scotch Brite® pad; then sand before re-coating.
Do not thin WEST SYSTEM epoxy; doing so will seriously diminish the epoxy's moisture barrier effectiveness.
4. Thoroughly sand with 220-grit sandpaper before application of the antifouling paint. Follow the paint manufacturer's instructions for final surface preparation and application.