I've clear-coated once over existing decals, no wet-sanding and simply masked off the decals, shot hobby clear from Michael's craft store, and then let it dry. I rubbed it out with compound lightly and it turned out OK, but didn't seem to seal and protect the decals all that much.

I had a frame painted, then decaled, then 4 coats of clear, and I doubt those decals will ever have a problem.

I'm pretty confident the DupliColor scheme works just fine.