I moulded a cored hull side and it came out looking perfect, but a month later you can clearly see the grid pattern of the contour-cut foam telegraphing through the gelcoat in the right light. It wasn't visible when I pulled it off the mould, so what's going on?
That delayed grid pattern is classic print-through from a contour-cut or scored core, and the timing is the giveaway. As a polyester or vinyl ester laminate cures and post-cures it shrinks slightly, and the resin-rich lines sitting in the saw kerfs between the core blocks shrink differently from the foam itself, so the pattern telegraphs to the surface over the days and weeks after moulding. Heat accelerates it, which is why it often shows up after the first warm spell. Prevention is the main cure: a thicker gelcoat, a print-blocking skin coat, or a layer of fine chopped strand mat behind the gelcoat to space the core away from the surface, plus letting the laminate fully post-cure before finishing. Filling the kerfs with a light filler paste before layup helps a great deal too. Once it has appeared, fairing and refinishing is the only real fix, ideally after full post-cure so it does not creep back.
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