Vintage Motorcycle Restoration: Aluminum Anodizing Removal & Repair

A. Hi, Jon. I don't have hands-on experience with such issues, but will give you some book knowledge until someone who has actually done it comes along.

For plastics, paints and clearcoats -- basically any organic finish -- Aircraft Stripper will remove it with no damage to the aluminum. However, this is methylene chloride based and is very toxic; rubber gloves ⇦ this on eBay or Amazon [affil links] , goggles ⇦ this on eBay or Amazon [affil links] , excellent outdoor ventilation, and every caution against spillage, splashing, and fume inhalation are mandatory.

For the surfaces which seem to be anodized (hard to scratch and non conductive), you'll want to remove that. In your aircraft company you could use a mix of chromic and phosphoric acid, but in your hobby you would be restricted to an alkali like oven cleaner ⇦ this on eBay or Amazon [affil links] . Unlike the acid, the oven cleaner ⇦ this on eBay or Amazon [affil links] will attack the aluminum substrate as well as the anodized coating, so timing is important. Again, googles, gloves, and care are required. If you do dissolve aluminum, that leaves copper and other alloying materials behind, which appear as a gray-black smut. In an airplane factory this is removed with a "desmut" step, but you won't be able to easily remove it with household products . . . I suppose you can try a mixture of vinegar ⇦in bulk on eBay or Amazon [affil links] and salt on your scrap parts; but don't overdo the Easy-off in the first place and you won't get smut.

At some point you do have to do mechanical finishing; you can't do it all chemically. Buffing is the easiest approach, although I suppose you could try a hobby rock tumbler [affil links] since you have spares to play with.

When you're done polishing, you need to clearcoat it. There are many approaches from simple one-component liquids you "paint" on, like Everbrite [a finishing.com supporting advertiser], which is fine but needs to be redone periodically, to 2-component spray-on automotive clearcoats. Good luck.

Regards,

A. I did this on my Virago engine sides last year as the plastic coating had become loose and tarnished. I tried various things, but finally stumbled on paint remover ⇦ this on eBay or Amazon [affil links] . It safely removes the plastic film and does not damage the aluminum. I then followed up with standard aluminum polish, and in the last year no problems have surfaced.

Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert

Also available in Spanish

You'll love this book. Finishing.com has sold almost a thousand copies without a single return request :-)

A. Thanks Herman.

Hi Terry. People come and go on a public forum, so I don't know if Herman stops by anymore. But it must be the nasty and noxious methylene chloride based paint remover, not caustic based paint remover, to be confident that it will not attack the aluminum. That's why I prefer to look for Aircraft Stripper, which is simply paint remover, but it's labeled so you know it's the type that's okay for aluminum, not the caustic kind.

Regards,

"How to Restore
Metal Auto Trim"
by Jeff Lilly

on AbeBooks

or eBay or

Amazon

(affil links)

A. Hi, John.

The usual result of glass bead blasting is a matte surface; in fact it's often done for exactly that purpose, so I'm curious why you would specify a finish that delivers the opposite of what you want :-)

Buffing or polishing back from a matte surface to full bright will probably take more than one polishing step. Letter 800 offers pretty exhaustive coverage of polishing aluminum. Good luck.

Regards,

A. I recently had the need to have a pair of '58 Zenith carbs, and mechanical fuel pump restored for an old BMW I'm doing. They are die cast; not aluminum and were they ugly; dark gray and stained. After glass bead blasting they were put in a vibratory tumbler with filled with ceramic media and water. After about an hour they look as good or perhaps better than the day they were made. The finish is smooth and clean, no blasting evidence. I have to think this would work excellent with aluminium parts.

A. Hi, Declan. caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] (sodium hydroxide) strips anodizing. Easy-off is just one commercially available form of it. Alkaline drain cleaning products, diluted, can be used as well. All of these materials are, however, dangerous and extremely caustic to human flesh. Don't go near any of them without goggles.

Regards,

Q. There are some more questions I need to ask. How dilute should the caustic soda be. How long should I leave it on the chassis. How should I apply it to the chassis?

A. Hi. Note that caustic also dissolves raw aluminum, not just the anodizing. I don't think untrained hobbyists should be trying to strip anodizing with toxic chemicals. So some some hands-on instruction by someone who has done it, and some practice with scrap parts before committing real parts to it, would be a very good idea; please don't use precious parts for trial and error learning. Good luck.

Regards,

A. Since this is an in-place strip, I would use a paste made of bicarbonate of soda and the finest scotchbrite pad sold with lots of rubbing. Calcium carbonate from a swimming pool place is a bit stronger. Both are slow but are far more safe than lye. If you must use lye, start out with a heaping tablespoon in 1 gal of warm water. Again, a very fine scotchbrite pad.

A. Hi Jerry. We attached your question to a similar posting where it was already answered. But you won't get to full brightness chemically. You'll need to do some buffing.

Regards,

A. There is a company in California that uses water and baking soda to blast the aluminum. It comes back looking absolutely AMAZING! Like it was brand new. I saw a clip on a television show where someone sent parts for restoration and the end result was the best I have ever seen. It removes every speck of grease, grime and oxidation. I am beginning the restoration of a 1965 Ducati 250 and will be sending them my engine case, cylinder, head and carb for this service. I don't remember the name of the company but Google should point you in the right direction. Good luck and happy restoring!
Tom

A. If it is anodized, the best stripping medium is chromic-phosphoric acid, but only an anodizing shop would have that. The problem with oven cleaner or any other caustic solution is that it attacks not just the anodized film, but the aluminum as well, so timing is important. You may be able to strip the anodizing yourself with oven cleaner, but I probably wouldn't :-)

A. Hi Aaron. Any caustic solution like oven cleaner attacks raw aluminum almost as readily as it attacks the anodized film. Yes, it is possible to remove an anodized film with caustic & careful timing, and some commercial shops operate with caustic as their anodizing stripper rather than using chromic-phosphoric strippers. But if left on too long, it's busy eating the aluminum. There is actually no "limit" to how much it can attack aluminum. In fact, a common metal finishing process to make complex hollow parts is to machine a solid mandrel/mold out of aluminum, electroplate it, and completely dissolve the aluminum mandrel away in caustic, leaving the plating as the object.

"Safely" is a problematic term; oven cleaner is fairly nasty stuff. Don't forget the goggles! But in industry, and even wearing full protective gear, no shop would let you near the stuff until you had passed a haz-mat training course; so when you ask what a consumer has to do to work with it "safely", you can see the problem.

A. Aircraft stripper will pull off a lot but it seems to be very patchy. I'm finding this out right now. I still end up sanding A LOT!

Hi, Aircraft Stripper is methylene chloride, which is a strong solvent, but still a solvent. So it removes clear coats and other organic finishes quite readily, but it's not the thing to use to remove anodized coatings. For that, chromic-phosphoric acid is best, as previously mentioned, and oven cleaner is probably second best.

A. You really should not have done that. The oven cleaner would not damage chrome or steel, but the aluminum is damaged (not stained). Perhaps the damage can be mechanically polished off. Otherwise you have to either live with it or replace it.

A. I've used oven cleaner for years to strip clear and colored anodization off my aluminum car parts to prepare them for re-polishing and re-anodizing. It shouldn't hurt raw polished aluminum, but if they were anodized, then yeah, you're screwed. The parts will have to be re-polished (use the oven cleaner again first to make sure all of the anodizing is off) then re-anodized if you don't want to keep polishing them by hand. BTW, aerosol cans of carb cleaner is my degreaser of choice. Some brands can harm some paint finishes, but it is generally harmless and cleans parts fantastically.

A. Be very careful when re-polishing and re-anodizing aluminum. If there are no critical features (threads or assembly fit features) then you will probably be okay. Anodizing is not just a coating. 1/2 your anodized thickness actually penetrates the material and removing anodize actually removes material that may not be able to be built back up in the re-anodize process.