I painted the floor of my race car with Rustoleum brush-on truck bed liner after I stripped out all the sound insulation. It's held up well except for a few high-wear spots that have scraped through. It is sort of a pain to apply; it's like trying to paint jelly. I hate spraying in the interior, since everything gets covered in a film no matter how well I mask.
I've used the spray-on stuff on the underbody, such as the rollcage mounting points and small rust spots, and that has held up very well.
I'd trust it to hold back rust better, but it isn't going to do much for sound insulation or strength.