I am looking for help.
After building several paddles and kayaks I have run into a finishing problem.
My usual procedure is one or two coats of System Three epoxy, two or three coats of spar.
On my last paddle the spar never cured and it stayed sticky. This summer I cleaned it with aceton, sanded, and reapplied the epoxy. I sanded and applied my first coat of spar.

Problems with finishes can be frustrating for me....and I've had a few over the years...

My suggestions:
a) Leave the epoxy to cure as long as possible before varnishing. Make sure you scrub the epoxy well with warm water and an abrasive pad (or wet sand with plenty of water). Rinse well with clean water. Purpose: to allow plenty of time for amine blush to form on the surface, and then to wash the amine blush off with water. Sanding alone will not remove amine blush. (I used a few gallons of SystemThree epoxy on a 'big boat' project and it was terrible for amine blush...but it just could have been the hot and humid weather interacting with an epoxy from the PacificNorthWest... ;-) )
An internet search on 'amine blush' and 'varnish paint not drying' will turn up plenty of fellow sufferers....

b) Do some tests with sample panels (scrap plywood) before you start applying finish to your boat - saves money and frustration. This will tell you whether there is a problem with the varnish - if it will cure/harden on bare wood, it is probably OK.
c) Are you thinning your first coat(s) of varnish? What are the drying conditions - temperature and humidity? I've found that varnish cures more quickly if I can get the project outdoors into the sun (once the surface is tack-free).
d) If your tests show that the varnish is slow to cure, adding Japan Dryers (or cobalt dryer from the art store) can help...but don't overdose the varnish...just add the recommended amount.

Please let us know how it works out.