Polyester resins mixed with hardener have to be above 65 degrees to harden properly.
If you lay Saran wrap or similar tightly over the uncured resin it will not have the "sticky" on the surface when first hardened.
I have been laying up some large fiberglass panels. To keep from having to race against the hardener to the finish. I have been laying up the parts in a 50 degree room and then bringing them into a 75 degree room after layup is finished. No more partially hardened polyester/hardener mixture on pert/equipment before the part is done.
I use a "waxed" gel-coat and resin to keep the "sticky" down. Then remove wax before finishing up the part.