I'm thinking about undercoating my truck this weekend. I don't have a sand blaster so I can't fully get all of the old rust off but I can get the most of it off using a wire wheel and a sander. After most of the loose rust and surface rust off, would you recommend coating it with rustoleum undercoating, bed liner, or regular rustoleum black paint?

 

Take it to the quarter wash, bring lots of quarters...... Power wash the underside of the truck and get it as clean as possible. It won't take off rust, but, it WILL get all the dirt, and loose scale off. Then use something like Chassis saver, or POR 15. Both products dry to a ROCK HARD finish..... You can't even sandblast that stuff off. (I've tried....)

 

Have a look at Fluid Film.

 

Once the frame has been cleaned, I apply Zero Rust (http://www.zero-rust.com/) - this stuff works well.