OT: corrosion repair on aluminum

I am preparing to have my car painted, which has aluminum trunklid and hood, both of which have a couple of spots of corrosion, electrolysis I think it is called(?). I have tried in vain to find an etching primer, or similar product which is formulated specifically for aluminum. I have been told that dissimilar metals will promote corrosion, which fact leads me to believe that etching primers containing zinc chromate are unacceptable, though the local body shop supply houses tell me that's what I should use. Anyone knowledgeable about this?

In my past aircraft mechanic days, zinc chromate was the primer of choice.....Use a respirator.....

Zinc Chromate. The pretty yellow/green.

Zinc Chromate is the aircraft approved primer for aluminum. Also, an alodine wash treatment is excellent as a paint base.

Zinc chromate protects the aluminum because the zinc will act as a sacrificial metal and corrode in favor of the aluminum. It's mandatory for seaplanes and aircraft floats to be zinc chromate primed on all surfaces.

BTW, the car wouldn't be an MGA would it?

I believe you may be looking for an Alodine solution. The product I have used is made by DuPont. I bought it in a 1 Qt plastic bottle.
It is used to prep bare (clean) aluminum for acceptance of primer on helicopters. Works well.

Thank you, eveeryone, for the info. Evan, the car is an old ('84) Mercedes Benz 300SD (I'm a Benz mechanic). I knew I could get reliable info here!

Zinc is about as close as you can get to aluminum on the galvanic chart:

http://www.thelenchannel.com/1galv.php

I also prefer zinc or cad screws and washers instead of SS on aluminum.

Nice car. The MGA also has aluminum bonnet and boot as well as doors.

BTW, the advice to use a respirator is good advice. Zinc chromate contains hexavalent chromium, a carcinogen.

First, you must remove all traces of existing corrosion via a STAINLESS wire brush. Using steel will just embed more dissimilar metal particles into the alumium and it'll corrode again very quickly.

Follow with an acid etch, you can use trailer brite or such to do this. Follow with Alodine, chromic acid or zinc chromate primer... but good luck getting the real stuff now, due to the health risk Evan mentions.

Try Aircraft Spruce and Specialty, Wicks, Wag Aero or any of the aircraft supply houses for your primer. There are several suitable etching primers available now in spray cans. Use the primer on the entire part or the paint will peel just about as quickly as it dries. You can go over this with an automotive primer to match the paint system you are using.

Thanks, Evan. Yeah, I really like the car... I've had it for around 6 years, and put about 100k on it in that time (now has about 250k on it), but still runs very well, I rebuilt the trans last winter, and just replaced a couple of body panels damaged by a car-deer collision. So now it's 3 different colors.
"Use the primer on the entire part or the paint will peel..." Why is this? And why if the whole part is primed with it will the paint stick?
I like your suggestion to use stainless wire brush... I knew not to use carbon steel wire brush; was planning on sanding discs, but stainless wire brush is a better idea. Do you think I'll have to sand after that, or just prime and then apply filler to finish?
Don't know where I'm going to do this, as it's about 28 degrees today and my shop's in the basement. Don't want to fill the house with toxic fumes... priming the whole panel throws a wrench into the plan too, as I was planning to clean the corrosion from the spot, and prime that spot. I'm trying to make some repairs here and there, as I can't afford to get it painted for a couple of months. You know, piecemeal as time and budget allow.

Paint doesn't generally stick too well to aluminum. Zinc chromate primer actually becomes part of the metal surface. It doesn't just adhere, it chemically combines with the aluminum. It hangs on so well that it doesn't even come off very well with paint stripper.

For example, I painted the removable cab top on my Land Rover with zinc chromate (Land Rovers are all aluminum) about 20 years ago. I didn't bother with any top coat. That paint is still in perfect condition.

It is definitely not something you want to spray in the house but it is also by far the best primer for aluminum. I worked in aircraft repair for years.

Any corrosion products such as aluminum hydrates (the white powdery looking stuff) can be quickly removed by wiping with battery acid diluted 2 to 1 with water. It will not eat the aluminum, only the corrosion products. Rinse well and even diluted it will still eat your clothing but isn't particularly dangerous to skin. Wear gloves and a face shield anyway.

Hi,

I would suggest that you strip all the paint off if there is any sign of corrosion. The best method is to use striper, you need to be sure there is no missed areas. Once the panel is bare, the wire brush is ok. On light surface spots bead blasting and Scotch Brite pads work well.

After you have the panel clean and all signs of corrosion are gone. You should use a metal prep and alodine. You can buy these at an aircraft supply or auto paint store. The prep is a citic acid wash, I use fine Scotch Brite to apply it. The alodine is the next step and should be done as soon after the prep as possible. The alodine is sold two ways, I prefer the type that tints the metal, that confirms coverage.

Once the panel is in alodine; you can use an etching primer or zinc chromate, I prefer using an etching primer. Vari-prime or an other good epoxy primer is a better base and the alodine also known as conversion coating protects the base metal. The etching primer is applied in thin coats and you may need to use a filler primer on top of the etching primer to ge a smooth surface.

One last thing; you said it is cold. I painted my airplane one winter here in Los Angeles, it was a record cold winter low 20's. You need to heat the area where you paint. Painting in temps below 65-70 are hard at best and below about 55-50 the paint will not cure. Heating the paint helps but the panels should be warm or the paint will not flow well. I made the mistake of learing this the hard and messy way. If you can't do the work until warm weather don't do any thing. Spray the areas that have corrosion with ACF 50 or other corrosion inhibiter and wait for the right time.

This is the method used in aircraft refinishing and quality auto and truck shops.

That said; I did not know the 123 and 126 sedans came aluminum hoods or deck lids. I know the 126 coupe and some 107's use aluminum for these panels.

Steve

Former M.B. Indy shop owner and driver 280SE3.5 C 300CE 560 SEC

As Evan said, paint just won't stick to aluminum, even typical auto primers. They are made for steel. Zinc chromate will bond to teh aluminum, which the primer can bond to, so you shoot your primer to that instead of bare aluminum.

You can probably spot it, but it'll be hard to hide it well if you want a real good bond because of the extensive prep. Just do what you can and plan on a total repaint down the road.

Notice most of the info here comes from the airplane guys? I work at an aviation museum.

You just think aluminum is a pain to paint, we gotta paint TITANIUM real soon here... LOTS of it (hint, hint). Still working on how to get anything to stick to that on something resembling a permanent basis.

Mike C sez,

"You just think aluminum is a pain to paint, we gotta paint TITANIUM real soon here... LOTS of it (hint, hint). Still working on how to get anything to stick to that on something resembling a permanent basis."

Well if it is what I think it is, just scuff it, clean it and spray it with an etching matt black. Buy a bunch of rattle cans to touch it up every now and then. I won't be going very fast or getting very hot. They are a lot of airplane to paint!

A long time ago we made parts for the Navy out of sheet titanium that had a coating spec. The finish looked a lot like green zinc or epoxy. It stayed on these parts and they were very flexible. They may have been oxided first.

Steve

Yep, it's what you think it is. In fact, you may have actually seen it... heheh.

So where do we get this "etching matte black"?

Sounds a lot easier than what I just got from a guy down at Eglin.

(sorry for the hijack)

I am not haveing any luck finding the actual stuff that was used. I did a quick search on the net and a DOD tech memo notes a wash used prior to finish wth glacial aceteic and phosphoric acids mixed with paint thinner.

I would try some Veri-prime, desoprime or any good self etching primer. The acid in the etch grips far better than zinc or other non etching primers. Some truck and bus companies now use etching primer instead or alodine at the factory and that paint sticks. One thing you do want to do is wash the surface with something that will remove all the oils and if possible lightly etch the base metal. None of the primers will be black, you will have to add tint and flatener to the second coat. The finish on the airplanes were rather flat from the Skunk Works and the cheap flat black spray cans are very close in luster to the original finish.

I was at Edwards 1993-1994 and there were two NASA birds there. I have also seen the YF-12A at Boeing and the one in Hutchenson. I also knew Darryl Greenamyer, one of the Locheed test pilots on the Sleed. A former sleed driver gave me some funny radio calls to FAA controllers that are a hoot to anyone that flys.

I am really sorry for hi-jacking this thread!

Steve

Thanks, I'll look further into this. Sounds better that the "shoot it with epoxy" I have so far.

This is A-12 #60937, the Skunkworks' former gate guard.

I had no idea my simple question would generate this much intrest! I don't know why I didn't think of aircraft techs as a source of info.

Steve, not all 126's have aluminum hoods, I don't think; the replacement hoods sold are steel (so I'm told by the body shop owner who the dealership deals with). And I don't think 123's have aluminum trunks or hoods. I'm sure they don't have alum hoods, not sure about trunklids.

Seems like I'm going to wait until warmer weather. I was planning on having the car painted in Feb, but the body shop which is going to do it is booked into March anyhow, so I'll wait until the weather breaks to deal with this. It's not as if there aren't plenty of other unfinished projects around here, and the corrosion isn't really getting worse, or at least not very fast, anyway.