No type of paint will permanently adhere to bare aluminum. To allow paint to adhere, you must prepare the surface by removing the aluminum oxide and chemically treating it to produce an appropriate substrate. Once you have prepared the aluminum, any type of paint will stick.
Painting Bare Aluminum
Clean the surface to remove contaminants. Power washing will remove most contaminants from the aluminum surface. Chemical degreasers added to the power washer will help achieve a clean, oil- and grease-free surface. You can clean smaller aluminum pieces by hand using a stiff aluminum brush, aluminum wool or a scouring pad along with a degreaser. Rinse thoroughly and allow to dry.
Abrade the surface to "roughen" it and help paint adhere. Grit blasting using aluminum oxide grit will abrade the surface quickly and provide a textured surface for coating adhesion. If a blasting unit is not available, you can abrade the surface with an angle grinder or sander using a coarse aluminum oxide abrasive disk or sheet. For small aluminum pieces, hand sanding with aluminum oxide sanding paper will achieve the desired result. Clean with compressed air and rinse thoroughly, then allow to dry completely.
Apply a thin, even coat of acid etch primer to the aluminum surface. Be sure to wear gloves and eye protection. After 3 to 6 minutes, rinse the surface thoroughly. Etch primers prepare the surface for proper adhesion by removing any trace amounts of remaining contaminants such as oil, grease or surface corrosion, and they provide additional abrading.
Wash the aluminum in a conversion coating immediately following the rinse after the acid etching, . Conversion coatings are available through most paint suppliers. Always follow the manufacturer's procedures and cautions. Chemical conversion creates a substrate on the aluminum surface that allows adhesion of coatings. Rinse the surface thoroughly and allow it to dry completely.
Apply a high-quality primer before painting. It is best to use one or two thin coats rather than one thick coat. Allow the primer to dry and then apply any type of paint; it now will adhere to the aluminum. Use a high-quality paint for best results, choosing exterior or interior paint to match the site.
Painting Previously Painted Aluminum
Clean the surface of the previously painted aluminum as described in Step 1. If the original paint is peeling, flaking or has lost its adhesion, you will need to remove the old paint to expose the bare aluminum, and then complete Steps 2 through 5. You can remove the original paint with a pressure washer or chemical paint removers. Chemical paint removers can be highly caustic, so always follow the manufacturer's safety warnings and instructions.
If the original paint is intact and sound, clean the surface as described in Step 1 and allow it to dry completely. You can now apply primer and paint as described in Step 5.