Aluminum is peculiar stuff.
How is your gun's aluminum frame finished now? Is it "bare metal" (that is, anodized)? Is it painted? Is it blackened? Is it black anodized?
What do you want it to look like, instead?
Bare aluminum exposed to air and moisture automatically forms a self-protective oxide which keeps it from further corrosion. Anodizing forms a much harder, sturdier self-protective coating, and can be colored, but it has to be applied by a specialty metal-plating shop. Paint wears off.
Brownells sells aluminum-specific spray paint. They sell sturdier, bake-on paints suitable for aluminum too. There also are proprietary paint and coating finishes, which have to be applied by a specialized shop.
Surface preparation is vitally important, in any case. You can hand-sand, sand- or bead-blast, or polish. Whichever you choose to do, it will affect the adhesion and final appearance of your re-finish job, so it must be done carefully.
Does that help?