I would go to town with a red Scotchbright pad by hand on those areas. It gets into recesses better than sand paper and you won't risk making the metal too thin. If you do use Ospho, make certain then you diligently scrub off the area to remove any excess dried on Ospho and the white powdery residue left behind. This powder is a byproduct of the chemical process. I use a red Scotchbright pad moistened with Ospho to do this cleanup. Again, the Scotchbright pad gets into the depression nicely.
This additional application of Ospho used for cleanup will not create more powder since there is to iron oxide present to make a reaction.
Clean up with your favorite degreaser before applying epoxy primer. I use acetone. Wipe it on with a wet lintless cloth and immediately wipe it with a dry lintless cloth. Do this until the dry cloth remains clean.
