Can you spray clear polyurethane over paint?

what kind of paint?? How long has the original paint been on there?

Many times, the "vehicle" (solvent) in the polyurethane will cause the existing paint to curdle or wrinkle if it is a water based/latex paint. If it has been on there a long time though, you might be able to get away with it.

See if you can test a small spot that is not exposed or visible first. Even if it "tests" OK, it still might have a tendency to pull the existing paint up when you get it on a large area, so be careful