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Finally pulled my 24 Outrage for the winter and did the annual hull wash with slimy grimy. Used a power washer to rid the hull of the nasty stuff. When I was washing the transom it blistered in several spots. Kind of star like blisters. Although the boat sat in the water for the summer, the bilge pump ran and the bilge was visually dry.

Can someone suggest why the transom would get blisters and nowhere else?

Boat is a 1997 and never had any blemishes in gelcoat before.

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Power washing itself can cause blisters. If you are not extremely careful when power washing you can force water into a tiny crack in the gel coat and the water will work behind it and fill and lift the gel coat until it explodes to relieve the pressure. I am not saying this is what happened but it is possible. I do not power wash my Whalers.

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Star-like blisers are seem more like stress blisters.

John

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I found one too, I think it's a stress crack, I don't think it's really a big deal, it's so tiny, I sometimes use a power washer, it's a very weak one (a bit better than a hose) and Chlorox liquid cleaner. to clean up in fall. You have to be careful (like Joe said), I did waste my new (solid core, ya know, the good one) VHF ant. with another power washer I used (a bit too powerful). I now don't use one, or I use a very under powered pressure washer. You can get amazing results with just some Chlorox liquid cleaner, brush, and a hose.

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I suppose it is possible that the power washer could cause the described damage, however it seems unlikely it would only happen on the transom when the entire exterior hull was exposed to the same pressure. Regardless, the damage is done. Now how to fix it? My plan is to let it dry out for a few weeks inside my warehouse, then lightly sand the areas until the blisters are gone. Backfill any void with gelcoat. Sand, fill, sand fill and buff. Any other suggestions?