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What is the difference between lamination and hotcoat in fiberglassing?

Laminating is the process of "gluing" the fiberglass to the foam with resin. It gives the board strength.

Hotcoating (or seal / sanding coat) it the process of applying resin with a brush on top of the fiberglass lamination to seal it from water and also providing a resin surface that can be sanded smooth.

The term "hotcoat" is an old term referring to glassers adding a lot of catalyst to polyester resin to make it cure fast. The resin heats up when curing and the board would feel warm - therefore a "hotcoat"...

Today with the popularity of epoxy resins, the term seal or sanding coat is more accurate and pertains to all types of resins.