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Hull Blisters

The hull of my 1981 Catalina 25 has three small areas, about one by two feet, covered with tiny blisters. Should I follow the treatment recommended in This Old Boat or do you have other suggestions?

Bob Schuetz

Don Casey responds:

First, the blister repair information in This Old Boat warned that it would be obsolete by the time the book was in print because it was written when the blister problem was relatively new and no single fix had been established as a true cure. Today I recommend the following regimen:

1. Grind open each blister, removing all damaged laminate (if any, since blisters often involve only the gelcoat) but not removing a millimeter more of the original lay-up than absolutely necessary.

2. Scrub out the open blisters with a solution of TSP (from any hardware store), using a stiff bristle brush. Flush the clean blisters thoroughly with water.

3. Allow the blisters to remain open as long as practical. It is important to allow the laminate to dry completely before filling in the blisters or you will trap moisture inside the repair, which will likely to lead to a recurrence. A simple test for moisture is to tape a piece of clear plastic over a sample blister in the early morning of a sunny day. Seal the plastic all around with electrical tape and make sure the sun will hit the plastic. If moisture forms on the inside of the plastic, the hull is still damp.

4. When the hull is dry, wipe each blister with a clean rag dampened with acetone. This is mostly to remove air pollution.

5. Epoxy is the resin to use for blister repairs. It is less permeable than polyester, or even vinylester, and it forms a much stronger bond. Wet out each blister with epoxy.

6. Now thicken some epoxy to a peanut-butter consistency with colloidal silica and fill the cavities with this filler, using a squeegee to compress and fair it. Silica-thickened epoxy is difficult to sand, so fair the repairs well. Never use microballons or any other hollow or absorbent (talc, for example) fairing compound to fill blisters.

7. Before the repair reaches full cure, paint it with at least two coats of unthickened epoxy. In your case, since the blisters are concentrated in three areas, I would coat those areas with 20 mils—about seven coats—of unthickened epoxy. The most effective application technique is to paint on the epoxy with a foam roller, then immediately squeegee the wet epoxy with a 1/3-section of foam roller. You make the needed squeegee by cutting a foam roller in half, then slicing the halves lengthwise into thirds. "Squeegeeing" the epoxy forces it into the pores of the hull and removes most of the bubbles that rolling has introduced, thus making the epoxy less permeable.

There is no benefit to returning the hull to its original makeup. After all, you already know that configuration is susceptible to blistering. Filling the blisters with epoxy resin will not lead to new problems, and epoxy barrier coats have proven quite effective provided they are thick enough—at least 20 mils.


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