Paint as undercoating?

Would spray paint work as rust prevention underneath? Why? Why not?

 

It would work while it was intact, which wouldn't be particularly long. Spraypaint is relatively fragile (brittle), and any flying road debris, even pebbles, can chip and scratch it at highway speeds especially. Any opening allows rust to start, which will then work under the edges of the damage to the coating.

Most undercoating products are flexible or waxy to avoid this issue.

If you need something easy to apply, consider Amsoil HDMP. I've been testing it on my truck and have been extremely impressed. It's available as an aerosol, and while it's slightly more expensive than e.g. Woolwax or FluidFilm, it's much more durable and doesn't need to be reapplied every year.

 

I'm thinking that I'll order this up and use it. I only hand wash my truck so it should last a long time