Manufacturers' instructions for applying plastic paints to masonry always seem to emphasize the need for a dry surface and dry weather. Just how important is this?
Very. In general it is always a good idea to follow any manufacturer's instruction to the letter since he, above all others, is anxious to get a result that will be a credit to his product. With the plastic (or catalyzed or reactive) coatings, the main effect of dampness will be on the gloss of the final coating. Hardening is dependent on a chemical reaction initiated by adding a catalyst or curing agent. This reaction is quite delicately balanced and the presence of water will upset it. The type of plastic resin also enters into the picture; a polyester resin is considerably more sensitive to moisture than any of the epoxy resins.