I have a relative in the paint business. He tells me that paint has two basic components - the resin and the tint. The tint merely gives color. The resin is a chemical formulation which forms a two-dimensional sheet as it 'dries.' Drying isn't just a matter of letting the water or thinner evaporate. It's a matter of allowing the molecules in the resin to connect with each other in all directions and form a sheet.

The main job of a guitar top is to bounce along with the sound, to resonate and make the sound rich and beautiful. It doesn't make sense to coat it with a solid layer of plastic and make it rigid. Possibly the effect would be minimal, but the only way to find out is to risk damaging the instrument. Not worth it, in my view.