Interlux just makes some great products, I'm impressed every time I use anything from them. I copied some of the info below on Watertite, mentioned by Don. I will note, blistering appears to come from several sources, reading some of David Pascoe's writings of the past. Some resins used by boat manufacturers over the years actually break down and become a liquid again. Thankfully, the resin Chris Craft used is about as good as it gets with fiberglass boat construction.
Interprotect Watertite YAV135 is an excellent fairing compound due to its unique two-phase cure, which gives a long easy-to-sand window. Interprotect Watertite YAV135 can be applied to a thickness of 3/4" without sagging and its superior spreadability makes it an ideal glazing compound for filling small nicks and scratches.
What is Gelcoat Blistering?
Osmosis is caused by a chemical reaction between water and unreacted substances remaining in the manufactured hull. Water breaches the exterior gelcoat and reacts with the chemical components creating acidic substances. These substances create pressure behind the gelcoat which causes blisters and eventually cracking. Once the gelcoat is breached in this manner, the underlying laminate is capable of absorbing water like a sponge.