Primer

If you sanded it all the way down to the metal, you should use epoxy primer first, then do your filler on top of that. You can do filler on metal like it's been done forever, but I prefer to do it over epoxy primer. After you get the filler work done, use some good 2K urethane primer to block sand the car with. I like SPI (Southern Polyurethanes) primers and clears.
https://www.southernpolyurethanes.com/