Body filler over epoxy
Applying epoxy to the bare metal and than applying the filler is always the best choice!
Mercedes and Porcha have required this for warranty work since the 70'S. Now six months ago Toyota requires it.
Here is why.
Body filler will stick to epoxy (unsanded) better than metal.
Body filler is not air tight and solvent s are never totally released (air and solvents equal rust) if they were it would crack. Have you ever sanded a ten year old body filler job and noticed the metal corrosion?
Another thing the epoxy does for a body filler is it makes it more flexible!
A word of caution: Never use filler over an acid etch type primer or a lacquer primer.