Products like Dura-glass and the fillers mixed with metals like aluminum are great for foundations before you apply regular bondo.
The duraglass is basically waterproof as is the metal fillers, but both should be skim coated with a bondo. They do not sand smooth enough to primer over as they leave many pinholes. That's why they should be skimcoated with a regular bondo.
I get the areas in bare metal ready by applying a coat of epoxy primer. The epoxies will accept any type bodywork over and the primer seals the bare metal against any moisture.