100 grit rule

I used 80 grit on my DA and board sander to remove the paint and surface rust, and to sand down any bondo. I went to 220. I used 2 good coats of primer, then sand with 400/600. I used a two bucket system, one with a teaspoon of dish soap and warm water and one with plain water for rinse; I used 1000 grit, just 2 passes from bottom to top. This smooths out the surface. Going from bottom to top keeps from sanding dirt back into the primer and the plain water is for rinsing then soapy water to sand.