Polyester Resins

Polyester laminating resin, with the hardener in the box

Polyester is the resin production boats are built from and the one most hull repairs should be done in. It is the cheapest way to put glass down, it cures fast enough to laminate several plies in a day, and it is the right partner for chopped strand mat. Quart through 5 gallon, every size shipped with catalyst.

General purpose or marine grade

General purpose orthophthalic resin is the workhorse for mold backing, filling and layup that is not going to live in water. Marine grade layup resin uses an isophthalic blend that stands up to constant immersion, and it is what belongs below the waterline. If the part gets wet and stays wet, pay the difference.

Waxed or unwaxed matters more than people expect

Laminating resin is supplied air inhibited, without wax, so each ply stays tacky and the next one bonds to it chemically instead of mechanically. That same surface will stay slightly sticky forever if you leave it as your final coat. Stir surfacing wax into the last batch only, roughly 1 oz per quart, and it cures hard and sands clean.

Tooling and surfboard resin

Isophthalic tooling resin is formulated for low shrinkage, which is what keeps a mold true instead of printing through and distorting as it cures. Silmar 249 is the clear surfboard resin, water white and UV stable, for glassing blanks rather than patching hulls.

Catalyst, measured not guessed

Polyester needs MEKP by percentage, and the right percentage moves with shop temperature: more when it is cold, less when it is hot. Too little and it never fully cures, too much and it gels in the bucket before you get it out. MEKP is stocked from 2 oz to a gallon, the MEKP calculator gives the dose, and styrene is here when you need to thin for spraying.