When you want to prep a metal part on your vehicle for paint, here are a few steps to follow.
Step 1: use your red scotch brite to scuff edges of the part.
Step 2: pre-soak 400grit sandpaper in warm water for 10-15 minutes. Then, while using plenty of water for lubrication, sand the large surfaces of the panel until you have a smooth, dull finish.
Step 3: wipe clean the panel.
Step 4: use your self-etching primer to cover any bare metal that has been exposed during the sanding process. Allow 20-30 minutes to dry.
Step 5: clean panel with surface cleaner (wet a paper towel with surface cleaner and keep a dry paper towel in the other hand, wipe with wet towel then immediately wipe with dry towel to avoid residue).
Step 6: open tack cloth and unfold then make it into a loose ball. With this you will gently rub the surface of the panel, removing any microfibres, dust or hair from surface.
Step 7: apply paint and clear as described in spray can paint process.