Primer

My opinion because I'm pretty partial to it, but SPI Epoxy primer is the best. Buy a whole gallon kit if you're going to do the whole car. It can be thinned to use as a sealer also. If you're in need of epoxy primer pretty quickly and don't want to wait on shipping, my 2nd choice would be PPG DP90LF, which you could get at a local PPG auto paint store. Whichever you use, do your filler work over the top of the epoxy. That way, rust won't form between the metal and the filler.
https://www.southernpolyurethanes.com/
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