Looked at an airplane the other day. Paint was okay but a couple patches were down to metal and could that could use touch up. These spots were not on the wings, rather, near the top of the back fuselage. Paint wearing and cracking. Could end up wanting to carefully sand and patch an area as big as 2.5 - 3 feet long by 1.5 - 2 feet wide. Actual bare patches are much smaller.

Looks aren't that important, at least, not as important as simply protecting the aluminum. Maybe I'll even get a mention in "Worst Paint Jobs"!

First, is it legal (or even not recommended) to do the work myself?

What do I use? Will automotive primer work for the base? What do I use over the primer, automotive paint?



Craig

Yes it is perfectly legal so long as you aren't disassembling control surfaces or the like.

Second, I am using self-etching primer and color-matched automotive coat from NAPA for my touchups. Like most painting it is all about the site prep and application.

Sanding is usually considered to be a very bad thing. Consider chemical strippers that are compatible with aluminum if you need to remove some old paint.

Next, clean and prep the metal. Read up on alumiprep 33. It contains an acid that's supposed to remove the corrosion and oxidation leaving clean metal.

Next, you will need to apply something to prevent corrosion. The problem with automotive primer is that it may or may not be compatible with aluminum. Read up on alodine. It can be used instead of a primer, and doesn't add weight.

Finally, you will need to apply some paint. Polyurethane paints like Imron are popular and long lasting, but if you know what type of paint was last used, you probably should use that type again. I once saw someone that had applied a decorative stripe using a different brand of paint, and it made the underlying coat bubble, dissolve, and run!

I believe it would be totally legal for you to perform this "preventative" maintenance.

You could use automotive primer but I recommend using Zinc Chromate. It's typically that dull green color and works well to help prevent the corrosion of the aluminum because of the zinc. It's $10-15 and comes in a normal spray can. It will be a difficult to impossible task to make the spots "go away" but at least you can help slow down the corrosion process.

If you are going to try and match paint I use the local CarQuest store to get a perfect match and then I apply it with a $200 airbrush so that I can blend it into the surrounding area with very little over-spray. This can be risky because you never know how the CarQuest solvents are going to mix with the old paint and you might get a nasty reaction, so start, very, very slowly. Spray a little bit and then let it sit a couple of hours. There's my two cents.

Matt Keller
http://blueiceaviation.posterous.com/

All above are very good answers so I will just add a few tips, Most aircraft and most marine aluminum is "al-clad" which simply means it has a very thin layer of pure aluminum on the surface of the alloy to protect it from corrosion. This layer can be taken off quite easily so sand very carefully or use a stripper very carefully. You can use the "scotch brite" pads, they work well and don't cut so fast, if you take off this layer the alloy underneath will corrode much faster.

You should use an acid wash to clean the area to paint, again careful not to get it on the other area,rinse well, then Alodine and rinse, then zink chromate primer, then paint.

One further word of warning, try to not use super slick paints that will not breathe, this is because when you do get some corrosion the aluminum will create gas, if this can bleed out through the paint all is well, if not you will get those funny little worm tracks under the paint. This is filliform corrosion, This is from the gas lifting the paint off and pushing along the skin.

Sorry to make is seem scary, it is not, if you can find out what paint is on the plane now you can use the same product and eliminate any problems that way, if you can't just lay some of the new paint on a small spot and see if it does anything nasty. One final tip, try to use the same brand with all the products, that way you will not have incompatibility issues.

If this seems to complex just get some rustoleum and a big ole' brush and slap it on there.

shorton wrote:
If this seems to complex just get some rustoleum and a big ole' brush and slap it on there.

Now you're talking my kind of repair work!

Thanks guys. One of the reasons I almost prefer a plane that doesn't have the greatest paint job, I don't have to worry about making it look much worse and I might learn myself something in the process.

A pilot is permitted to paint an entire airplane EXCEPT for balanced control surfaces. Only an A&P can paint balanced control surfaces, and the reason for that is that paint can disturb the balance of the control surface, which can result in flutter.

Wings and fuselage--Go for it....

MTV

shorton wrote:All above are very good answers so I will just add a few tips, Most aircraft and most marine aluminum is "al-clad" which simply means it has a very thin layer of pure aluminum on the surface of the alloy to protect it from corrosion.

My understanding is that the top layer of pure aluminum is a sacrificial layer. It oxidizes immediately and protects the alloy underneath from further corrosion. After all these years, especially on planes that were polished, how much of that pure aluminum layer remains?