As Bill said, epoxy takes weeks to finish curing at temperatures in the 70s.

Each epoxy is a complicated mix of chemicals which may interact with the mix of chemicals
in each varnish, differently.

One of the first kayaks I built more than 20 years ago has varnish on the inside which never cured properly, because in my ignorance, I used whatever marine varnish my local paint store had available.

The manufacturer of your epoxy should know which varnish will be compatible with that epoxy, so it will cure properly.

The first coat of varnish on epoxy less than a month old, always takes longer to cure. Additional coats should cure in 24 hours and be ready for re-coating.

Uncured varnish can be removed easily with steel wool and alcohol.

It's hard to be patient when you're so close to the finish line of your project, but I'd suggest you scrub down the existing varnish with steel wool and alcohol, to remove any uncured finish, and wait for warmer temperatures to apply multipul coats of a compatible varnish. Find out from the epoxy company which varnish they suggest. In the meantime paddle your boat and have fun.

Good luck!

Rob

As usual, wise words from Rob!

From the WEST (Gougeon) website:
http://www.westsystem.com/ss/problem-solving/#paint-varnish

PROBLEM: Paint or varnish will not set up over epoxy.
POSSIBLE CAUSES & SOLUTIONS:

Epoxy not completely cured.

Allow the final epoxy coat to cure thoroughly. Allow several days if necessary for slow hardeners at cooler temperatures. Apply moderate heat to complete the cure if necessary. See Controlling Cure Time in the User Manual.

Paint incompatible with epoxy.

Use a different type of paint. Some paints and varnishes may be incompatible with some hardeners. If unsure, test for compatibility on a coated piece of scrap material.
Use 207 Hardener. It is compatible with most paints and varnishes.
Epoxy surface not thoroughly prepared.

Remove the amine blush and sand the surface thoroughly before applying paints or varnishes. See Final Surface Preparation in the User Manual.

Epoxy resin/hardener combinations vary in their tendency to develop amine blush.

WEST Slow 106 Hardener was one of the worst; 207 one of the best in a test here:
http://www.oneoceankayaks.com/Epoxresl.htm#blush

Note that the surface should be washed to remove blush before sanding. Wet sanding is also a good idea.

I agree with Rob and Bill's advice to not overcoat non-cured finish, and preferably to remove the uncured material.
In the (now-distant) past, I piled on varnish coats a couple of times and ended up with a sticky mess.