One of the earliest boating memories I have of my father is watching him patch up “Old Greenie,” our old 12-foot Sears and Roebuck Ted Williams skiff. And he wasn’t just patching it up, if I’m honest. He was basically reattaching the forward port and starboard sides back together at the bow. But that’s a story for another time.
Back then (in the early 1970s), boaters had pretty much one resin to choose from when it came to boat repairs: polyester resin. Today, those choices have become a bit more confusing. Head into any marine supply shop and you’ll be faced with a myriad of choices including polyester, vinylester, and epoxy resins. So which one’s best to use for what? Let’s clear up the confusion right now.
Polyester and Vinylester Resins
Polyester and vinylester resins are both styrene-based and catalyzed using methyl ethyl ketone peroxide (known as MEKP for short), which kicks off the hardening reaction. Vinylesters are generally more resistant to water absorption than polyesters, which is why they are often used in hull blister repair. Both are very strong (most all fiberglass boats today are molded and built with one or the other); have good adhesive qualities when applied to properly prepared fiberglass; and are affordable for most boaters (though vinylesters do cost more).
Epoxy resins are different from polyester and vinylester resins in more than a few ways, but the easiest to recognize is the way in which they’re catalyzed. With epoxy resins, it’s always a 2:1 ratio of resin to hardener, but you use different “speed” hardeners to make it cure more quickly, or more slowly. Use a “slow” hardener when it’s hot outside or you need more working time, or a “fast” one when it’s cold outside and you need the repair to cure ASAP. Epoxy resins have great resistance to moisture absorption, abrasion resistance, and excellent adhesive or “gluing” qualities. One downside to epoxy resins is the price; they are about three to four times more expensive than polyester resins. Hey, all of those good qualities have to cost something.
Picking the Correct Resin
Generally speaking, when you’re doing a fiberglass repair, the adhesive qualities of the resin you’ll be using are a prime consideration. In this sense, polyester and vinylester resins are pretty adequate for most do-it-yourself fiberglass boat repair jobs, such as laminating fiberglass cloth into holes or cracks, or reinforcing an area (such as a stringer or transom knee) with woven fiberglass. Thickened polyester resin is also great for making cosmetic, non-structural repairs such as gouges and screw holes. For general fiberglass repairs, it's fine to stick with good old affordable polyester.
All that said, epoxy resin is much, much more tenacious when it comes to adhesiveness, and it “sticks” great to boats made of polyester or vinylester. But (and this is a big “but”) polyester and vinylester don't adhere well to epoxy, so if you plan on applying gelcoat to finish the repair (gelcoat is a type of polyester resin), make sure you use polyester. If you try to apply gelcoat to epoxy, you won’t be happy with the long-term results. Epoxy resins also leave behind a slimy coating called “amine blush,” which needs to be cleaned off before you attempt to stick anything else to the surface.
You’ll of course want to dig in to all the ins and outs of surface preparation (mechanical and chemical) before you start any fiberglass boat repair project, but knowing which resin is the best for your particular job (and budget) is almost half the battle. Because unlike my dad and old Greenie, today's skiff repairers have choices.