What does your artist friend say about the surface? Besides the issues with the tooth of epoxy for any given media, your friend probably has specific requirements for what the media goes on, otherwise you would probably risk problems with adhesion and lifespan. Primers are used because they're good at sticking to things, and not all media are. I'd ask before you wed yourself to a plan. It may turn out that as long as you're going to put a clear over it and not another layer of epoxy, primer would be better. Also check to be sure that whatever clear coat you use is compatible with the media used for the artwork.

I like graphite/silica mixed. It is tougher than the clear coat I have used. I'd recommend contacting the manufacturer of your epoxy to get specific instructions for mix ratios. If you put in too much powder you'll lose the strength of your epoxy, and that isn't quite what you're after...

m