I use this for any rusty stuff thats solid or difficult to remove. Its the best I have found. I buy in gallons from industrial supply place. Any place that sells loctite products should have it. Make sure you follow the directions closely though.
http://www.acehardware.com/sm-loctit...i-1274208.html

 

So, this stuff can be used for body applications? Or is it for bolts and stuff?

 

I use it on anything thats rusty. Its a chemical converter. Goes on white like milk, turns purple then black as it cures. Surface can (needs to have) rust to chemically cure. Use 2 coats then top coat with paint or under coating. Superior to any of the "rust" paints available. It is thin to so it wil run and creep into inaccesible places. I first found it at work where they use it on piping and equipment that is exposed to salt water spray. It works!
It must go on bare metal that has some rust though, the oily coating on new body metal must be removed. Try it.

 

When I can get around to actually doing some body work, I will. Thanks for the tip. Sounds good!

 

Rustoleum sells a similar product called "Rust Converting Primer" in spray cans. Works good - converts rust to a good solid primer.

 

I second the Extend atta boy. Used it on a weird window corner once. Primed with high build primer and sanded and primed again a few times till smooth. Couldnt see the repair and it never came back in the 5 years I kept the car. Great stuff!!
Jim