I used to to body work way way back when I was just a wee one, so take what I am saying with a chunk of salt......but....
I remember using a self etching epoxy primer that had to mixed with a catalyst and that stuff was the bom! Smells like bubbe gum and was a greenish gray color.
I also remember the color of the primer also effected the holdout of the final paint job depending upon the color you were using such that you wanted to match the color as close as possible to the primer, but this is not a huge deal unless your painting like a final coat a white color you would not want to use a red primer for example.
So yeah I agree with wolfman, look for a self etching and preferably an epoxy type with catalyst.
Good luck