If your epoxy isn't totally cured, the varnish will not cure either. Thing is, epoxy can look and feel cured, but still be kinda green. The previous advice about letting it cure for a week is good. I had one paddle where I let the coat of epoxy cure for two or three days, seemed pretty solid, but once the varnish was applied, no joy in Mudville. After a week, it was still tacky, so it was back to the sander to start over.
As per the above consensus I wait (at least) a week after a final epoxy coat and then test sand (dry sand) a small area to make sure I’m getting dust and not rubbery goo. I’ve rushed that, more than once, and having to remove the goo and start over takes longer than having waited a couple of extra days
Shop temperature and humidity matter, and having a concrete slab floor doesn’t help. The cold sink makes it harder to heat in the winter and raises the humidity level in the summer. In sweaty summer weather I need to run a window AC unit to drop the humidity level.
The winter solution that works best for me is running an electric radiant oil heater. Set on low 600W under a gunwales down canoe the hull captures enough heat that the shop may be cool but the entire boat stays warm. I use that same radiant oil heater to help dry/cure epoxy, hanging parts and pieces over it.
P3020013 by
Mike McCrea, on Flickr
About thwarts, yokes, carry handles and such, I finish them before I finish them, meaning that I dry-test fit them in the canoe before any epoxy or varnish coats go on. In part so that I can pipe cleaner some epoxy or varnish inside the machine screw holes (drilled 1/32 larger than the hardware), but also so that I am assured everything fits perfectly. Plus that provides a machine screw hole to hang off a finishing nail.
It sucks to get epoxy or varnish on and then discover that, oh crap, I need to sand this end down just a wee bit more, removing those finish coats and needing to re-apply them.
About sanding epoxy coats, I dry sand test a small obscure area, but tend to wet sand (220 usually) almost everything, including brightwork. In part to help remove any possible amine blush or contaminates but mostly because I have too often/too easily cut all the way through my finish coats when dry sanding, especially with too much vigor when using an RO. I’ve cut through even hand sanding with 120
Provided my epoxy coat(s) are smooth and free of drips and sags I’m just looking to take the gloss off the epoxy before a varnish or paint coat.