I'm patching a section of laminate below the waterline on my yacht and I've got polyester resin left over from a deck job. Would it be fine to use that down there, or is epoxy genuinely worth buying for a repair that stays submerged?
For work that stays submerged, the resin choice genuinely matters. Epoxy absorbs noticeably less water than orthophthalic polyester and bonds better to cured laminate, which is why it is the usual choice for below-waterline repairs and osmosis rebuilds. Vinyl ester sits between the two and offers much better water resistance than standard polyester, so it is a common step-up where an epoxy budget is not available. Using leftover polyester below the waterline is a false economy, since it is the material most prone to the water uptake that leads to blistering in the first place. For the submerged patch, reach for epoxy resin, or vinyl ester resin as a middle option, and keep the polyester for topside and deck work where it performs perfectly well.
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