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Dealing with Blisters.

First off, epoxy is the cure-all chemical for hull and deck repairs; don't even waste your time trying to find polyester resin.  More often than not an epoxy repair done right is stronger and more durable than its polyester predecessor.

There's a lot of history behind this.  In the 70s and early 80s many boat builders thought polyester resin gel coat to be the most foolproof material for fibreglass boats.  Many years and thousands of osmotic blisters later they discovered that polyester resin was neither foolproof nor, more importantly, waterproof.  For the trailerable sailboats it worked fine and is still used in many of the smaller boats today, but for boats mostly slipped in the water polyester resin just wasn't the answer.  Sometime in the early to mid-80s many larger boat builders began shifting to vinylester resin. 
Q - Ever wonder why people apply a barrier coat and an anti-fouling bottom coat to a boat below the waterline? 
A - Polyester resin.  The barrier coat repels moisture away from the polyester.  Vinylester hull doesn't need a barrier coat.

There are two basic ways to deal with blisters. 

1. If the blisters are confined to a specific area then sand the tops off, fill the holes with epoxy and bottom paint over them. 

2. If the blisters are spread overall then take all the gel coat off with a disc grinder using #12 grit resin disc.  Apply one coat of Interlux 2000 epoxy and fair with fairing compound.  Apply two more coats of first coat on the bottom and finish with two coats of the finish coat.  You can apply the epoxy with a paint roller and finish with a foam brush to smooth the surface. 

NOTE: Take extreme care with anti-fouling paint.  Wear a mask.  Capture the ground off dust and dispose of it as toxic waste. 

I have done several of these repairs and find the Interlux system the easiest to use.  The hull must be dry which will involve hauling your boat out of the water.  Drying it under a well ventilated tarp for a month or more is not uncommon.  In very hot weather, you may be able to coat in two weeks.  On the west coast we plan for at least a month.  Use a moisture meter to accurately measure water content.  Don't guess at this.  You can also tape a piece of plastic wrap over a section when it is cool and see if condensation forms when the sun comes out.  If condensation appears, you need to dry more. 

Links for blister repair;  www.yachtsurvey.com/my_wet_hull.htm
 

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