Corrosion Clean-Up

[David Heal]

I am considering the purchase of a '53 C170B. I have owned/operated many small planes over the past 42 years, so I am not a total newbie in this regard. This particular C170B was partially stripped (of paint, that is) a couple of years ago and hasn't been used much since. The bare Alclad surface is now covered by the usual surface corrosion (spots of surface corrosion here and there, all over the surfaces) -- the corrosion pockets are not real deep, but all over. I will, of course, give the bird a thorough interior inspection for corrosion (skin lap joints, inside Alclad, forgings, non-Alclad parts, etc.) as well.

In 25 words or less, what is involved in cleaning up this surface corrosion and prepping the surfaces for painting? Does one use something like Scotch-brite pads and a "cleaner" to manually clean-up each of the surface corrosion areas? Followed by some kind of wash or conversion treatment?

Many thanks for any insight. -- David

[GAHorn]

Common paint strippers (methylene chloride types) will take care of/remove superficial surface or "filiform" corrosion. Alodine (if you never plan to polish) and then paint. Otherwise, prime and paint. Do not recommend Scotch brite unless very severe surface corrosion exists.
Never, never use steel wool or steel/stainless brushes on aluminum.

[bsdunek]

My 170, after sitting too long, developed a lot of corrosion spots on the top surfaces. The place I had paint it, Sturgis Aviation (Michigan), used synthetic abrasive rotary brushes to clean up the spots. They come in a variety of grits, from 36 to 220, and are used on a rotary tool like a pneumatic drill motor. They leave no metallic residue themselves.
After treating all the spots, a chemical treatment was used to assure there was no more corrosion, and then the usual painting techniques were used. I have been very pleased with their work.

[c170b53]

Be wary and avoid generating excessive heat when using mechanical methods such as rotary scotch pads on an angle drive. In my opinion prepping by hand using scotch pads with water and alumi prep poses no danger to skins to prepare the surface for primer prior to painting.

[David Heal]

Thank you to all for your suggestions. We'll see where this goes! -- David

[misterde]

A method I used a number of times...

1. Strip paint...I use "Aircraft Stripper" by "US paint" I think.
2. Wash surface to remove all residue
3. After totally dry. Scrub areas using scotchbrite pads with "Alumiprep Acid Etch" mixed & applied as directed. Corrosion should be entirely removed at this point. The acid is not strong but it will remove the oils in your skin & will cause extreme dryness & cracking of skin on your hands so protection is needed. This process eats away the corrosion as you scrub open the corrosion. You are not removing good aluminum as long as you do the next step.
4. Rinse with fresh water very thoroughly to stop the acid action. This is really important.
5. After airdrying by mechanical blast air or just open air drying , Apply "Alodine" conversion coating. Instructions are on the container. Basically it is brush on & keep wet for 10 minutes, rinse off with water and let it dry by itself(no air blasting or fans here). It will leave a very thin conversion coating of a straw color on the aluminum that paint will readily stick to. This coating is very fragile and is meant to be recoated and I think the instructions say within 24 hours (there is a time limit).
6. Prime & paint

Wear rubber gloves through the entire process and none of it is good for your health so read the cautions and dispose of responsibly. Do not use this process if you are going to polish bare aluminum & leave it bright. This is the accepted process airlines use on their jets which have severe corrosion problems. All materials are sold by "Aircraft Spruce" and DuPont auto paint dealers under different names.

Dick DeCramer
'49 C170A N9007A
Northfield, MN

[HA]

RE: AlumiPrep and Alodine - this is the final thing you do before priming and painting, BUT it won't remove anything other than light surface corrosion - the stuff you don't even see. If you're dealing with any visible corrosion from filiform on up, then it will need to be mechanically removed before any treatment.

After the corrosion is removed you need to check that the thickness of the skin wasn't compromised beyond limits - aircraft service manuals will have the limits (Although I'm not sure about the "100 Series" manual that applies to these airplanes) OR look in AC43 1 and 2.

as a guide, if you have flaking corrosion (intergranular, the worst) then you'll be replacing that section so don't bother trying to save it. light surface corrsion can usually be removed safely. if there is a LOT of corrosion then I'm worried about inside the skin lap seams, disassembly is the only cure there. but that's pretty much worst case scenario.