David,
In my experience, the chalk that I think you are describing is due to UV
exposure (the sun). Epoxy coatings should be covered with several coats of
UV resistant varnish. I think that Epiphanes has some of the highest
resistance. The chalkiness is not just on the surface but all the way
through the epoxy. If this is just on the deck and other exposed surfaces
that have no structural strength value, just sand with 220 grit and paint
it. If you want to finish the wood bright, you are going to have to sand
down to wood and recoat with epoxy and then apply 4 or more coats of
varnish.
Charles