If you're painting bare aluminum, even in patches, you must use and etching primer or it will come off. Treating the aluminum with muriatic acid will also work but I don't know the process. Aluminum pores are too small to hold paint, even after sanding, which is also something you don't do to aluminum. Scotchbrite pads in different grit are commonly used. Red is coarse, green is more medium and white is fine. Done right that paint will be there for a long time. If you have an auto paint supply nearby it would be a good idea to pick their brain about the newer products. Some paints are designed for aluminum and will etch themselves.
Creativity is just intelligence fooling around