[lawspud]

[I posted this question in the RV-12 forum, but thought I'd post here as well to reach a wider audience.]

So, I pulled out my first wing skin from the box where it has sat for about a year (I know, I know...). On the inner surface I noticed some black-ish discoloration:

The discoloration is smooth (no apparent pitting) and there is no apparent residue. It's just...there. (I'm referring to the zebra striping. There is also a reflection of my unlit overhead shoplight in the aluminum, which I'm not worried about.)

How should this sort of thing be addressed? My guess is that it is oxidation that I can polish out and prime. But that's just a guess.

Any input from the more experienced out there?

Thanks

 

[chaskuss]

No worries

Jeff,
From your photo, that appears to be minor. Simply polish it off using a maroon hand ScotchBrite pad. Real men own a right angle die grinder and a Roloc attachment mounted in it. If you own one of these [see below] use a blue ScotchBrite disc to buff that discoloration off. See

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=93088

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=99559

The tools shown above are great time savers for deburring holes and polishing edges.

Charlie Kuss

 

[D-Dubya]

Or you could try...

As goofy as it sounds, we once cleaned some schmutz (not corrosion) off of some QB wing skins with a Mr. Clean Magic Eraser. That was used when nothing else would work, including MEK, alcohol, etc. Just make sure the skin is clean--no dust or dirt on the surface--then wet it with water and Mr. Clean might help you take it off. Dry it with a soft rag when you're done, and you won't have any scratches on the Alclad surface. Might be worth a try.

 

[apkp777]

Don't use the maroon scotchbrite. You will remove the alclad. Just polish it off with a cloth. A buffer would do a good job with some aluminum polish.

 

[Phil]

If it were me, I'd scotchbrite it down and then rub on some alodine. It'll be covered with paint later and they're going to scratch and alodine your skins anyway.

Phil

 

[Flying Scotsman]

Just a suggestion...before doing anything, maybe identify precisely *what* it is that you're seeing? Was this by any chance in areas were the sticky blue vinyl stuff wasn't stuck (i.e., bubbled up?). Or was the vinyl fully stuck down and this was under it?

You can always polish/buff stuff out, but I'd want to know what it is that I'm looking at (and what caused it) first...

 

[lawspud]

Just a suggestion...before doing anything, maybe identify precisely *what* it is that you're seeing? Was this by any chance in areas were the sticky blue vinyl stuff wasn't stuck (i.e., bubbled up?). Or was the vinyl fully stuck down and this was under it?

You can always polish/buff stuff out, but I'd want to know what it is that I'm looking at (and what caused it) first...

And that is certainly one of my questions.

Yes, this was found in an area where the plastic had bubbled up and left a gap. The stripes seem limited to the area that was exposed by the bubbling (that is, it doesn't seem to have "grown" beyond the bubbled zone). I have found this same striping in areas where the plastic had bubbled on two other skins. None of the discoloration seems to have eaten into the aluminum, in that the surface is not perceptably pitted or eroded. In each case, the area affected is in the preformed bend of the leading edge. I'm guessing that a small amount of moisture or condensation accumulated there at least once, in spite of the fact that the skins have been stored under cover.

The stuff didn't seem to respond to MEK/soft cloth. Maroon scotchbrite scuffed it right off, leaving no visible discoloration behind. I'm manly enough to have an angle grinder, but not quite manly enough to have the Roloc attachment...maybe someday I'll be big enough to own one

So, I'm not sure what the stuff is, but it doesn't seem aggressive and seems consistent with surface oxidation or staining in an area where the protective coating separated from the skin. I'll drop a photo or two to Van's for their consideration, but I'm optimistic at this point.

Thanks for all the input. Please speak up if you have any insight.

 

[terrykohler]

Don't use the maroon scotchbrite. You will remove the alclad. Just polish it off with a cloth. A buffer would do a good job with some aluminum polish.

Jeff:
The alclad provides additional corrosion protection for the aluminum sheet. Why grind it off with scotchbrite? A proper paint job will include surface pre-treatment which usually involves a mild etching process as well as a barrier coat such as alodine. The latter also contributes to corrosion resistance. Stop sweating and keep building. Just $.02 from a retired metallurgist.
Terry, CFI
RV-9A N323TP

 

[chaskuss]

snipped
The stuff didn't seem to respond to MEK/soft cloth. Maroon ScotchBrite scuffed it right off, leaving no visible discoloration behind. I'm manly enough to have an angle grinder, but not quite manly enough to have the Roloc attachment...maybe someday I'll be big enough to own one
snipped
Jeff

Jeff,
Glad to hear that you were able to remove it. I learned about the Roloc attachment and discs from my RV guru. He's a 30 year A&P, IA who does sheet metal repair for a living. He told me that these attachments are a "must have" tool. He says that professional aircraft sheet metal mechanics don't use the swivel deburring tool provided in most of the tool kits. It's to slow, hard on the wrist [when doing a lot of holes] and opens up the possiblity of removing base metal from the part.
The ScotchBrite Roloc discs come in 4 grades:

Carmel = Coarse only for use on steel
Maroon = Medium for use on thicker aluminum. Use for reducing deep scratches
Blue = Fine This is what you want to use for deburring and edge polishing
Gray = Extra Fine Not needed, we are not polishing gold here!

This tool will really save you time and effort.

Charlie Kuss
PS I did a test with the blue discs to see how long it would take to break through the Alclad. You have to hold a brand new disc in one spot for about 3 to 4 minutes to break through. Breaking through the Alclad looks similar to breaking through the finish paint into primer when you wet sand a paint job. You can see the edge of the Alclad.

 

[lawspud]

Jeff:
The alclad provides additional corrosion protection for the aluminum sheet. Why grind it off with scotchbrite? A proper paint job will include surface pre-treatment which usually involves a mild etching process as well as a barrier coat such as alodine. The latter also contributes to corrosion resistance. Stop sweating and keep building. Just $.02 from a retired metallurgist.
Terry, CFI
RV-9A N323TP

Well, while I agree with this in principle, these areas are on the inside of the skin. So I need to address this particular issue before installation rather then after.