What have folks been doing to fix or prevent all the rusting that happens underneath the car? Trying to fix/restore a project car and its pretty bad.

 

For what proper rust repair can run you'd probably be better off finding a rust free car from outside the rust belt. If you have to replace floor pans, trunk pans, suspension components, lower sections of body panels, etc its really expensive. As in more than the car is worth to fix properly.

Rust isn't an issue here in north Texas, both my cars look pristine underneath.

 

Have you guys tried using 3M Pro Grade Rubberized Undercoating or Cosmoline RP-342?

 

My 04' 125k G was from PA and it had rust everywhere. My 05' 150k G was from TX and doesn't have 1 spot on it. Rust repair is a tricky thing, you have to really get in there and do it right or it's just going to haunt you later. Honestly I wouldn't spend the time and money to repair rust problems as cheap as the 03'-08' cars are getting.

 

I only rust repair vehicles I plan on keeping forever, my GMC Sierra is pretty thrashed all over with quite a bit of rust on the bed but I'm definitely not going to be fixing that rust... I think... maybe I'll just buy another bed someday

 

That's good to prevent rust in the future but if you already have rust it won't do much. Certainly won't fix it.

 

Since these cars are getting old, its not worth fixing the proper way (replacing the metal).

The best bet is to slow the rust down, by sanding off as much as you can, prime and paint it to keep it protected from the elements. Wash the car frequently in the winter.