A lot of trim pieces on older cars were made of aluminum that was anodized. Anodizing is an effective way to create a hard, weather resistant surface that will last a number of years without tarnishing. Eventually, however, the metal surface becomes stained and scratched and looking generally dull, so much so that it either needs to be replaced or cleaned.
Unfortunately, anodized surfaces don't clean well. In fact, if you scrub or polish such a surface all you will get is a shiny drab-looking finished product. In order to properly clean and polish aluminum you have to de-anodize it.
Sounds difficult, doesn't it? After all, the anodizing process requires acid baths, cleaning steps and electric current, not to mention timing equipment and dyes (for those colored surfaces like brackets, etc.) One would think, therefore, that de-anodizing would require the same materials, only in reverse order.
That would be nice and effective, but it turns out that aluminum is one of those metals that can be chemically de-anodized. All you need is the right chemical, and you can find it in your grocery store. The "magic" chemical is sodium hydroxide, and it is the active ingredient in drain cleaners (Drano). It comes in liquid and crystal form and we find the dry crystals are the most effective (and least expensive.)
All you need to de-anodize your piece of aluminum is a shallow pan large enough in which to lay it. You'll need warm water, rubber gloves, eye protection and, of course the drain cleaner. Fill the pan with enough warm water to cover the piece and then add enough drain cleaner to do the job — we find that 1 tablespoon of cleaner to one gallon of water will de-anodize a couple pieces like headlight bezels.
Mix the cleaner thoroughly in the water and place the aluminum piece in it. Make sure there's air movement over the pan, since the vapors can be a bit caustic. Watch the bubbles form on the aluminum and lift it out every minute or so to remove the accumulated "smut." Smut is the chemical residue that forms during the de-anodizing process and you want to remove it periodically to expose the surface to the chemical. If the process is going too slowly you can add more cleaner.
After a few minutes you will see that the piece is a uniform, flat color. Stains and blemishes should have disappeared, leaving only scratches. Take the piece out and rinse thoroughly with fresh water. Dry it off and inspect for any residual anodized spots, which show up as darker surface finish. If there are any, immerse the piece in the chemical for a little longer and rub them out. Rinse and dry again.
Now you can sand your piece with 800 grit paper to remove scratches and then buff the surface to a bright shine. It will remain a soft surface unless you anodize it again, but that takes equipment and a little experience. We find that keeping a coat of wax on the piece prevents tarnishing and staining. Also, you can spray the piece with clear lacquer or enamel to protect it in harsher environments.