Todd, yes you can use the primer on painted surfaces after sanding those surfaces first. And you can also use the clean metal primer instead of the rusty metal primer like I did. The reason I used the rusty metal primer is because I have some rust issues on my beater and even though I sanded the rust away, this primer is supposed to help seal it.

Peck, the biggest reason to use primer over sanded painted surfaces is to provide a uniform surface to paint over. An example would be my truck. It is currently at least 5 different colors and each color would cause the paint to appear differently, especially with the first thin coats. If it was all one uniform color, the paint will cover more evenly. Also, the key is to not sand down to the base coat.