Repainting Aluminum Castings

Question:

I have a number of aluminum castings to repaint. I would like some advice on the best way to strip and repaint cast aluminum surfaces. The castings were originally painted with baked-on black enamel. Moisture must have gotten under the original paint and caused oxidation of the aluminum, which has caused the paint to start chipping off. I assume that mechanical removal of the oxidation is the proper way to clean the surface after paint removal. After paint and oxidation removal, what would be the best primer and paint application to use for a durable painted surface? W. T.

Answer:

First, you must remove the remaining coating from the aluminum castings. Chemical or mechanical methods are the best way to strip coatings from aluminum, since thermal methods are usually too hot for the metal. If you use aqueous chemicals, you must remove the water before painting. Abrasive blasting using the correct media is an excellent stripping method for aluminum. The choice of media depends on the toughness of the coating. One good choice is the plastic media that is used for stripping aircraft coatings without harming the aluminum skin.

Traditional surface pretreatments for aluminum include deoxidizers, phosphoric acid-activated vinyl wash primers and chromates. If the abrasive blasting does not remove all the oxidation products, the aluminum must be deoxidized using aqueous chemicals. The water must be removed before painting. The castings can be recoated using one-coat baking enamels or powder coatings. Alternative finishing systems would be: an epoxy primer topcoated with a polyurethane enamel or a phosphoric acid-activated vinyl wash primer coated with an epoxy and then topcoated with a polyurethane enamel.