I think there's a few variables here.... mostly regarding the quality of the finish you're after? If this hardtop is anything like the ones I'm familiar with, there's a "grain" as well as ridges embossed into the surface when the alum. sheet is made into something you'd use on a roof. It's not something you're going to sand down smooth, so don't even consider that! I would think anything that would work on the aluminum trim/siding on a home would work fine? A lot of people have used Rustoleum products on the above deck sides? Just make sure it's clean, and maybe go over it with a Scothbrite pad or similar to prep? For something with a little higher quality finish, the sky is the limit. Acrylic enamel, basecoat/clearcote, etc Just don't let anyone talk you into an epoxy based product. That stuff doesn't handle the sun well at all.... yellows like crazy.