Another trick with bond. If you mix a small amount of fiberglass resin in into the Bondo it becomse more like a spot puddy. Not great for forming but great for finsh coats. Also the fiberglass is a sealer. The hybred bondo/fiberglass goes on better but it does not sand better. My 8" DA can male quick work of it but if you are using a long board it will take a little longer.

I use about 75% bondo and 25% fiber glass and the normal amount of hardener you would use is the whole amount was bondo.

 

Have you ever used the "removers" that pull the oxygen out of the rust, They are amazing. They will not hurt the paint. Below is a part off a 68 pick I was doing. It is a inside door support. It is funny to how much of the original paint was hidden by rust. To bad you can just dunk the whole car in this stuff.

Before






After

 

What kind/brand of rust remover did you use there? I remember from way back (ok....a few years) when you were working on a pickup truck. Whatever happened to that project?

 

Oh yeah, all the Bondo this, grind that, and rust remove/convert the other thing talk made me forget to mention one thing....
The acid in etching primer can cause adhesion problems with body filler. So if you're wanting to primer first, filler second, you'll want to apply epoxy primer over the etching primer before you slather on the filler. Whether or not you use etching primer, you only want epoxy primer to be under filler, if priming-filling is your plan. This is only important if you want your (hopefully) quality paint job to live a good life.

 

The above part was done in evap-o-rust. You just soak it overnight. There are several companies that make this type of remover. Even Eastwood make one.
Rust Remover - Eastwood Rust Dissolver

Truck is a long story, I had to move due to a plant closing, new neighborhood would not allow unregistered or disassembled cars anywhere but in the garage so I had to choose between my 68 pick or my 67 Camaro. Not really a choice even if the truck was finished. It’s a shame too cause I have just found a rusted out 1981 Suburban with a 3 year old crate 427/Turbo 400 with the gear vendor overdrive. It was a friend’s race car tow rig. That would have been a great shorty truck. I ended selling the whole project, because of the suburban I broke even.

 

Another thing to keep in mind with rusty panels is if the rust is enough to have pinholes! Sometimes they're so tiny they are not visable to the naked eye, and if not found they will wick moisture from behind and create all sorts of ugly problems in the future.
It's always better to cut out any metal that is deeply pitted, as it may be so close to going through that it will not be noticeable if there are tiny holes.