IF you are dealing with a painted surface, a coat of clearcoat over the top is all you would need. When we paint scale hydroplanes, we always install the decals either before or after the first coat of clear and then clear over the top to protect and seal the decals. One big difference is we run fuel that is 50%+ nitro and 16%+ oil so it's a whole different animal than the less than 20% fuel used in aircraft