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After laying down your fiberglass and letting it cure, you'll notice a sticky residue that will remain on top that will gum your sandpaper every time. I find it best to wipe the fiberglass first with thinner prior to sanding to keep the sandpaper from gumming up.
Thought I'd share.
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I saw this post, and even tho it's older, I thought I'd add my .02...
Make sure the polyester resin you finish with is just that, a Finishing Resin. If you still have a tacky surface once the resin is apparently cured (given enuf time) it's possible you've used Laminating Resin, which stays tacky to accept the next layer of resin & 'glass cloth. As stated above, you can clean it with acetone perhaps, or lay a coat of finishing resin over it.
Hope this helps. however belatedly.
DaveA