My understanding is you can not paint over enamel with epoxy paint(adhesion issue).
Here's the break down on most paints, varnishes and epoxies:
Paints are solvent based, and yes, water is a solvent, just a mild one...
Paints with volatile solvents, (just about all solvents except water), may or may not be compatible over an old painted surface. If the old paint is a one part, (no catalyst or converter needed to cure), just about any new one part paint can be used over the old. That's about 98% true, so test out any new paint over the old before going whole hog.
What any old single part paint will not be compatible with is any new 2 part paint that requires either a catalyst or converter. The catalyst or converter is usually too hot and will destroy the old one part paint, with the exception of a water based old paint.
Epoxies, on the other hand are not solvent based and the hardener is amine. Amines don't react with cured solvent based paints and epoxy will readily adhere to all old paints as long as the painted surface is properly cleaned and scuffed up with sandpaper, (80 - 220 grit dry).
Adhesion, for any new paint or epoxy over old paint or epoxy is not so much the adhesion of the two, but how well the old paint or epoxy is adhering to the substraight.
So, it's OK to paint most one part paints over old one part paint. It's OK to apply epoxy over old paints of any variety. It's not OK to paint a two part catalyzed paint over a one part old paint. If a two part catalyzed paint is to be used successfully over an old one part paint, an epoxy barrier must be applied between the two paints.
Epoxies, when cured, usually produce an amine blush on the surface. Amine blush is the result of incomplete cross linking between the epoxy and the amine hardener. This can happen with inadequate mixing or excess hardener in the mixing ratio. Either way, before painting over a new epoxy surface, the amine blush must be removed completely.
Fortunately, amine is water soluble and the blush can be removed by scrubbing with soap and water, followed by a fresh water rinse and light scuffing after the surface is dry. Amine will act just like a mold release agent if not removed.
As far as varnish is concerned, clear coating with a linear polyurethane like Awlgrip is a fairly common practice, especially for teak transoms. The key to not destroying a beautiful varnish job is to apply two thin, but complete, coats of clear epoxy over the varnish. Then remove any amine blush and apply the PL clear.
In the end, 95% of adhesion problems can be traced back to inadequate or improper prep....