The easiest and least expensive would be DupliColor Self Etching Primer #1690 available in spray cans at O'Reilly Auto parts stores. I believe it's $7.00 a can. It is the light olive green color. Clean first, sand with 400 for some "tooth", spray 2 coats, let dry overnight, sand with 400 or 600 before you put your color on.
I used this product in 2004 as primer on aluminum race car trailer fenders before single stage black, to this day, still there, except the black is somewhat faded since it sits in the sun all day.
Realized I forgot something, in using this paint, make sure you spray it outside, if in a closed in area, you may feel some burning in the eyes, it has some type of acid in it, hence, etching primer