If Metal-Prep is phosphoric acid based, be sure that the epoxy used is tolerant of that. Even washing down with water may not get rid of all the iron phosphate, which most epoxy doesn't go well with. I don't think you'd need to go as file as 220 or 320 grit with your sandpaper. Generally rougher is better for paint adherence.


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.