Just a guess, I would try spar varnish in a spray can. Home Depot sells Minwax brand. I use it on paddles. A light mist would seal the artwork without causing it to bleed. You can then build up coats. then you could steel wool the surface and coat It with epoxy for a more durable finish or stay with the varnish. Don't forget, you have to protect the epoxy with varnish anyhow.