The cause of blistering is well known
I read an article years ago from a guy who had been the chief engineer of Pearson Yachts. He said the biggest cause of blisters was the production methods. When they first started building fiberglass yachts, they really did not understand the materials all that well. Many times they would start to lay-up a hull at 4pm, work until quitting time and then stop and finish the next day. Other times they would start at 0800 and finish laying up the hull that day. More often than not, the hull started at 4pm and finished over several days would exhibit blisters. What they found was that after spraying the gel coat into the hull, if when it was still tacky they sprayed the first layer of resin, then glass, then resin on and on until they were done - each time starting the next layer when the previous layer was nearly set, but still a little tacky it had much less blisters. In this way they found that the layers bonded together and made a better water resistant hull.
Very interesting article, and a really cheap fix for boat builders.
