Ok, this is from my Paint Guy, I prepped for him for over 15 years in a Custom Paint Shop. he`s a cool knowledgable cat. And he`s does my Nitro for my guitars now and then.
" you`ll prolly need to use a 2 part catalyzed Clear.... automotive stuff. You need to bridge carefully between the oil based on the surface and the thicker clear coats. You do this by applying/spraying 3 VERY light mist coats onto the surface. You go light mist coats 1st because of the thinners in the Clear. Too heavy and the Clear coat thinners will re-activate the oil based surface of the artwork and it`ll be mush. So a light coat 1st misted on. Wait a bit to allow it to set up/flash off. Let sit for a bit. Add 1 more light mist coat and wait till it sets up. Then 1 last light mist coat of Clear. Light mist coats allows the solvents to flash off waaaay faster than heavy coats obviously.
When these coats feel to the touch fairly dry, and your surface artwork looks intact still, you should be able to add some heavier coats. The early 3 misted on Clear coats will be a safety bridge barrier that protects the artwork from the later heavier coats thinners/solvents as they flash off. Get me? If all goes well, you should be able to build up some clear coat layers, that once cured/dry.... will allow you a good leveling base for a smooth finish, and the artwork will stay intact. "
I hope this may be of some help.