It depends on the epoxy you are using, but ideally you need to fillccoat before the previous coat has hardened. The point of fillccoats is that they fill the weave so that you can sand prior to varnishing without abrading the cloth fibres.

Some epoxies will still give a chemical bond for a fairly long time after they appear to have hardened, but Blush seems to set in as the curing process goes from tacky to smooth. It is much worse if the day is cold and damp.

In my experience, fast hardeners and medium hardeners are much more prone to blush than slow hardeners.

I have experience with West and Mas. Other epoxies will surely have their own quirks, but as a general approach...

I keep my workshop as warm as possible during an epoxy session.

I regard coating the hull or deck, from wetout coat to final fillccoat, as a single operation.

I keep a hawkeye on each coat and overcoat while the previous coat is still a bit tacky.

Even if I have to stay up all night, I get all my coats, wetout and fill coats, on in one long session lasting all day and half the night.

Using Mas epoxy, with slow hardener, which can blush, but is not prone to, I find only about three to four hours is needed between coats. I never let the previous coat reach a state where it is no longer a bit tacky.

This approach has kept me blush free for years now, although Mas epoxy and A warm workshop are, I think, mostly the reason, because I don't seem to get blush any more, even as the final coat cures.

I have a theory that very low viscosity resins, like the Mas, can pull away from a hardened surface, if it is not keyed, through lack of resistance to the effects of surface tension, leaving small uncovered dots. I don't believe ths is blush related. This is a secondary reason that I overcoat early.

Et