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[December 8, 2005]


When I finished up last night, something was bothering me. When I looked at leading edge on the inside of the skins, some of the ribs made some pretty nasty scratch marks in the aluminum. This had be concerned for a couple of reasons - the first is could this lead to a crack? The second is since the alclad is long gone, is it worthwhile to prime it. So when I got into work I posted some messages on Yahoo's RV7and7A message group and here is what I got back:

Original Post:

Hey guys, Last night I was taking the vinyl strips off of the the inside HS skins and noticed in a few places some scratches in the skin from where I mated in the nose ribs (not large scratches..no more than 1/4"). The scratches definetly go through the alclad and there is even a small amount of material displaced from the scratch. A couple of things have me concerned. 1) Would this sort of scratch lead to some sort of structural failure? 2) What is the best way to smooth out the scratch without taking too much of the alclad off? 3) Once I smooth the scratch down, is it a good idea to clean it and then spot spay it with spay-can self etching primer to make up the the lack of alclad? BTW, I am not planning on priming the inside of my skins. Thanks! Mike Bullock http://www.rvplane.com




And the feedback:

Mike Those sort of scratches do not hurt at all and will occur throughout the project. Any scratches should be smoothed or rounded out. Sharp edges and burrs create conditions for cracks to start, where a rounded edge does not. A Scotchbrite pad will usually suffice on alclad scratches. Use a soft Scotchbrite wheel in a polisher or die grinder for deeper scratches & deburring edges, etc. I am priming all, so my scratches are protected. I would suggest that you touchup as you mentioned after smoothing. My .02 worth. Bill Swaim RV7 Slow Build Fuselage N615KS Reserved ------------------------------------------------------------------------------ I think this is quite common and are really probably pretty insignificant. That said, you do need to sand the scratches out, or at least round out the scratch. You should also prime any area in which the alclad has been removed--a rattle can primer should be just fine. Unless they are really deep I don't see where it could possibly be a structural issue. BTW, you'll see this same thing when you get to the wing leading edge and fuel tank. Even when you do a good job of smoothing the ribs before inserting them into the skin, you have to push them in so tight that the first time in they will probably scratch a little. Good luck, Steve RV9-last part of wings ------------------------------------------------------------------------------ Knock the little burr off, go over it with a scotch brite pad then hit it with a little primer and you'll be just fine. I would'nt worry so much about removing a little alclad. It will not be the last time... Darrell ------------------------------------------------------------------------------ Everyone has a few scratches here and there, it's impossible to not have any. I would smooth out any scratches that can be felt with a fingernail using emory cloth and a scotch brite pad. Don't worry about the alclad, you are most likely through that already, it's only about a thousanth thick. Clean the area good with MEK, Coleman Gas or some other cleaner that leaves no residue. It's clean when there is no longer any "black" on a clean rag used for wipeing. Then, shoot the area with primer. (primer will not stick to shiney aluminum very well, but will stick after the scotch brite treatment). Take this for what it's worth. Larry ------------------------------------------------------------------------------ Mike, I had the same thing happen to me. The nose of the ribs dig into the skin just a tad making the scratches. I learned to look for anything protruding from ribs and such that might cause such a problem and bend them in or run them over the scotchbright wheel. Unless the scratches are really deep you don't have anything to worry about. Your on the right track about needing to smooth them and prime. You might be able to get in there with a right angle die grinder with a roll-loc scotch pad. Or just hit it with some fine sand paper and work your way down to say 320 or 400. Then spot prime with a self etching primer. I've had great luck with the Sherwin Williams or Napa self etch primer in a spray can. (I've been told they are made by the same company) Vans has some good info on this in your preview plans book. Bruce Smith




When I got home, I used the above recommendations to get the skuffs out of my inner skins. The emery paper followed by the scotch brite pad worked great. Took about a minute per scratch. The biggest issue was working in the nose of the skin with trying to small pieces of emery paper to not scratch off too much of the alclad.

After that, I was able to debur the holes on one skin. The inner holes near the leading edge were a pain to get to. I ended up removing the deburring bit from the device and spinning it by hand.

Here is a shot at the scratches inside the skin. When I saw them yesterday they looked way worse than they did today. Funny how that works.


Another shot. I didn't know how well my ghetto digital camera would take these close up pics


Here is the end result. I needed to cut back some more of the vinyl because there was some scratching in the vinylized part too here. A little emery paper, some scrotch brite and it was done! I circled it with a sharpie to make sure that I would remember to spot prime it when I took the rest of the vinyl off.

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