Almost anything, though I'd avoid polyurethanes. Specialty woodworking stores will sell lacquer for spraying (comes in paint cans instead of aerosols) which was pretty much made for spraying. And it dissolves quickly in lacquer thinner(which is also what you'd thin it with, so it's not going to ruin your equipment if it dries. "Polycrylic" water based varnishes are good for spraying, but they kind have a fakey look to them. You'd thin them with water. Unless you're spraying foam, I'd go with lacquer, as it looks best, and is easiest to clean up if things go awry. But be sure to test compatibility first- you don't want to spray a coat of lacquer over an armor piece, then come back and find something that looks like alligator skin.