Blistering (image #7)
What: Bumps on surface of the fiberglass
Where: Mostly the hull below the waterline
Why: Low-quality resin, gelcoat or laminate
Severity: Low to medium
Fix: Costly and time-consuming grinding and recoating
Blisters are bumps in the finish, shown above (image #7) after being ground off. Blister size and location reveal much about the cause of the underlying problem. In most cases, blisters develop due to the eventual absorption of water into the boat’s hull shell or deck laminates.
Very small blisters (less than one-eighth inch in diameter) usually have a source close to the surface. Larger blisters (one-quarter to one inch) normally have a source deeper within. Excreted brown fluid may also be seen, especially if the blister is punctured and pressed. In most cases, blisters do not lead to any structural problem. They do hint at the use of lower-performance materials and/or problems with the laminate’s quality.
Several types of polyester are used to make gelcoat. Less-expensive polyesters can hydrolyze after long-term exposure to moisture. In this process, the chemical chains that make up the hardened resin break.
Back in the early ’80s Everett Pearson (of Tillotson-Pearson) and Interplastic Corp. collaborated to solve this problem. I performed all the testing and dissection. We discovered that, if a vinylester resin was used immediately behind the gelcoat, blistering wouldn’t occur. This was only true if the vinylester layer was of sufficient thickness and the gelcoat was an “NPG isophthalic” polyester.
Most boatbuilders after the early ’90s have used this technique to build blister-free boats, but many still opt for lower-cost systems that run the risk of blistering. My advice: Make sure you’re getting a hull and deck made with a vinylester “skin coat” (first layer behind the gelcoat). The repair of blistered locations on hulls can run to $1,000 per foot.
It is not just the gelcoat and skin coat that count, but also the quality of the laminates behind them. Fibers must be completely wet out with resin. Air bubbles need to be rolled out. Both trapped air and dry fiber can be a source of blisters. This is even true of vessels with a vinylester skin coat.
If your boat’s manufacturer recommends not keeping your boat in the water (yes, some actually state that), consider Interlux’s InterProtect system, a two-part epoxy developed to prevent “boat pox.” Available to boaters, it’s also the first choice of many pros when preventing or repairing blisters is the order of the day. yachtpaint.com