What is a proper grit to prepare "A" faced ply (birch if it makes a
difference) for epoxy coating prior to painting.

You could sand the surface with something im the 180 - 220 range, maybe even
coarser, but if the surface of the wood is natural, clean and smooth, I believe
you will get a good bond between the wood and the epoxy even if you don't sand.

Almost all my experience is with the Gougeon brothers WEST System epoxy
(http://www.westsystem.com/). However, I believe several of the other epoxy
systems are similar. WEST was developed initially for coating wood surfaces..
the WEST stands for something like Wood Epoxy Saturation Technique.

On plywood or wood, I have put two or three coats of WEST epoxy on. You can
either put on the second and third coats once the previous coat is hard enough
not to run, or you can wait until it cures and sand it. This sanding is both
to get a good bond between the epoxy coats and to smooth the surface for the
next coat. I believe the WEST system literature says that if you recoat within
24 hours you do not need to sand the previous coating.

I believe that if you are coating the surface while it is horizontal, you may
be able to get a thicker coat on without it running, and get away with only one
or two coats.

The instructions that come with the paint you are using should tell you how to
prepare the epoxy surface for painting. Normally I would let the epoxy cure,
and perhaps leave it a few days to be certain I have a complete cure, then sand
with something in the 220-400 range to get a nice smooth surface but with some
texture for the paint to stick to.

Some epoxies, under some circumstances, get a coating called amine blush as
they harden. You are supposed to wash and or sand this off before putting any
more epoxy or any paint on the surface.

Good luck,

Alexander Meller