Blister, Blister, everywhere a Blister...
Gelcoat blisters generally occur between the gelcoat and laminate, and are typically .039" (1 mm) to .39" (10 mm) in diameter and .019" (.5 mm) in depth. They are often filled with a caustic, watery liquid.
Cases may be mild, with random blistering, or the blisters may completely pepper the bottom of the boat. Areas close to the waterline and about the turn of the bilge are most frequently affected by blistering. Most blistering affects only the gelcoat, although there have been instances where damage has penetrated the structural glass fibers of the hull laminate. (See bottom, Figure 17)
Once started and left unattended, glass fiber delamination may spread as moisture wicks along the glass strands. The moisture reacts with contaminates trapped within the polyester laminate accelerating osmosis process, resulting in inter-fiber delamination. Therefore, it is a good idea to repair the affected areas when blistering first appears, regardless of how minor the first occurrence may be.
Four basic criteria must be met when repairing blisters:
1 The hull must be thoroughly dried. Applying a barrier coating to a wet laminate will only compound the problem.
Note: A "Moisture meter" needs to be used to verify hull dampness. A perfectly good hull that has been in the water for a year or more may take several weeks to dry out enough to do epoxy finishing.
2 Blister damage must be excavated and repaired with a quality, structural adhesive such as WEST SYSTEM Brand epoxy.
3 A minimum of 20 mils of WEST SYSTEM epoxy and 422 Barrier Coat Additive should be applied to the hull below the waterline.
4 A high-quality, marine bottom paint should be applied; and the integrity of the bottom paint and epoxy barrier coat maintained.
As osmotic pressure increases, gelcoat blisters between the laminate and polyester gelcoat begin to appear.
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