the idea of sanding back to bright metal is to protect the metal from moisture. If you sand the metal or surface with 80-150 grit, epoxy primer will bond. If there is existing paint that you can't get up, then you can epoxy prime over it but you will be stuck with any problems that the old paint has itself or has covered up. You can apply the surfacer without sanding the primer if coated within the proper window or sand with 200-300 grit. Here is a list of pad color to grit I found online.
SCOTCHBRITE GRIT CHART
3M Scotch Brite Nylon Pads:
7445 - White pad, called Light Duty Cleansing - 1200-1500 grit
7448 - Light Grey, called Ultra Fine Hand - 800 grit.
6448 - Green (?), called Light Duty Hand Pad - 600 grit
7447 - Maroon pad, called General Purpose Hand -320 grit
6444 - Brown pad, called Extra Duty Hand - 240 grit
7446 - Dark Grey pad, called Blending Pad 150 grit
7440 - Tan pad, called Heavy Duty Hand Pad - 60(?)
Blue Scotch-Brite is considered to be about 1000 grit.
Mike
Last edited by Rabaut; 04/15/2023 10:36 PM.