Painting.

[WorkinMan]

The different color in primer is just that, different color. As someone mentioned you should only use black when painting a darker color. Essentially it will help your base coat cover a little better. Self-etching primers are meant for use on bare steel. The "etch" part of it is what helps it grab ahold of the steel. If you're painting over anything else any 2 part primer is will work and you can fill any and all sand scratches with the 2k and then just block it out. After primer you shouldn't use any grit rougher than 320. I personally try to stick to using only 600 or red scotch brite. As far as that chrome I have a feeling that you're not going to have much luck getting things to stick to that, at least not for any amount of time. If you want this to last and don't want to buy a new grille I would suggest trying to rip that chrome off of there with some 80 grit, then shoot some 2k on it, block it and go to base.

Just my two cents though.