Roger,
No, not the same. Polyurethane is solvent based and is fuel proof. But, it will yellow slightly over a light color (white, light blue, etc.). And it is heavy. Polycrylic is water based and not fuel proof. It is commonly used as a sealer or, mixed with talc, as a build up primer. It must be top coated with a fuel proof product, but since it's water based, almost anything will cover it with no compatability issues. Oh yea, since it is water based, it might warp a thin sheet balsa constrution. I've used it for years and never had a problem, but others apparently have.
Paul