Number ONE most important thing about POR15 is that it NEEDS soemthing to bite to. Rust is perfect, but if you want to coat new metal or previously painted surfaces, you have to prepare them properly. New metal can be prepped with their Metal Prep, which gives it a slight etch, or it can be sanded with 80 on the DA, or hit with the grinder (though I'd still go over with the DA to make sure it's evenly scuffed). Painted suraces have to be well-sanded. I'd go with 320.
