Fiberglass by era: blisters, resins and cores

Boats by era, 1975–2010 · Power & sail

Fiberglass boats of the 1970s and 1980s were built mostly with low-cost polyester resins that let water in, and blistering became widespread. Builders moved to more water-resistant resins and later to foam cores that don't rot, but not all at once and not all builders.

The blister years

• By the early 1980s blistering was widespread and repair yards were learning by trial and error. Research in the late 1980s and early 1990s traced most cases to water reacting with soluble ingredients in the resin and glass mat, and to weak bonds just behind the gelcoat.
• A boatyard manager wrote in 2004 that nearly all older boats were built with orthophthalic polyester resin and gelcoat, the cheaper, less blister-resistant kind. Binders used in glass mat in the 1970s and early 1980s were another culprit.

When builders changed resins

• There was no single switch-over year. One New England builder was using a blister-resistant vinylester skin coat behind the gelcoat by 1985. The 2004 writer said the move from ortho to iso resins came sometime in the 1980s and varied by builder.
• Even in 2001, a former builder's engineer estimated only 15 to 18% of US boats used the better resins (more among sailboat builders). A 1990s or 2000s boat is not automatically blister-proof; ask what its builder used.

Balsa cores, then foam

• Many boats of the era have balsa-cored decks, and some have cored hulls. Balsa is light and stiff but rots when water gets in through unsealed holes.
• Core trouble wasn't only a 1970s problem. A Canadian surveyor recalls well-publicized core saturation in some late-1990s and early-2000s models from one large US builder, traced to unsealed cutouts.
• BoatUS says production builders widely adopted synthetic foam and composite cores, which resist water and don't rot, by the early 2000s.

What it means when you shop

• For any boat from this period, ask the owner and the model's owners' association about its blister and core history.
• Ask whether the bottom has been peeled and relaminated or barrier-coated, and get the paperwork. A documented, warrantied repair can make an older boat a better buy than an untouched one.
• Where a boat lived matters: boats kept in warm water year-round tend to blister more than those hauled every winter.

Sources disagree on Year builders switched to blister-resistant resins — left out rather than guessed. Check your local authority.

General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.