^^^I've found this to be true.

But to the original problem; shooting it two times may have contributed as well. If you do that, you are for sure going to get a "lip" on the edge of the masking—even in a best case scenario. You'd have to sand, at least the edges, of the masked paint to get a clean release and then clear coat it. Tamiya paints (I think) don't have near the viscosity of typical spray can enamel.

I'm still screwing up rattle can jobs myself. The small Testor's cans of spray paint seem be be a bit thinner and have a better spray for smaller jobs, but then, of course, the little cans cost as much or sometimes more than a standard can of Rustoleum.