Paint would work on the exterior, but what about the innards? If the finish is well worn on the outside, the interior's not going to be like new. I suspect paint on the inside would jamb things up pretty good. If you just paint the outside, then how are you going to prevent the guts from corroding on your wet hunts? You'll probably rely on a liberal application of a fluid or aerosol protectant. Why not use the same stuff on the exterior? You say you want the paint to prevent rust, but I suspect you're looking for a cheap dress-up, and there's nothing wrong with that.
In reality, most shooters will apply a water disbursing agent inside and out before and after wet hunts, and you'll do the same.
If you're trying to duplicate Remington's classic bright blue with spray paint, you'll be disappointed. But it you're looking for a matte or camo finish you may be well pleased with paint. Have you considered camo tape or a ghillie wrap to pimp-up your field gun?
