Right on rustbucket.
Eastwoods Rust Encapsulator is good stuff. Just brush it on chassis/pan work and it self levels pretty well. You can buy rattle cans or spray it too. I had to remove some for welding and it took a lot to get it off.
For body work, have the rust stripped and painted with an epoxy sealer. Then filler, high build primer, paint. Careful who you have strip the paint -- you could end up with warp panels if they sandblast incorrectly.