FWIW When I had my frame powder coated I sanded the chipped areas till the frame was smooth if there are slight dents in the aluminum where the paint is chipped the sandblasting will not take them out so the metal needs to be smooth. It really doesn't take much effort with a 150 or even 100 grit paper. The blasting will take off the paint and rough up the metal well enough for the powdercoat to adhere better.