You'll probably have a compatibility problem with the poly. The question arises as to whether or not the poly you are using is impervious to the solvents in your paint. If so, how long does it need to cure? There is also the question on whether or not your spray paint can mechanically adhere to the poly. If it's glossy, it probably can't. Then there is the matter of weight; foamies are usually light planes, so 6 coats of poly is a significant addition. I think you'll need to find a single coat solution to make this work.
Rustoleum isn't that bad. It's not hard to touch up when it does get scuffed.