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Thread: Paint Stripping and prep for repaint question

  1. #1
    gdafoe's Avatar
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    Paint Stripping and prep for repaint question

    Not a super cub question but, I Looked a a couple airplanes recently that had been stripped and being prepared for new paint. What are the rules regarding sanding universal head rivets? How far down before it is not airworthy? Maybe 10% of the rivets still had dimples many where down half flat and some even lower. Not really wanting opinions here but real standards, if there are any. What I remember from A&P school is you don't sand it at all, if the dimple is gone you replace the rivet.
    Gerald

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    wireweinie's Avatar
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    I don't know of an actual reference but I had a former fed tell me about a C-172 that had been sanded like that. It ceased to be an airplane at that point.

    Web
    Life's tough . . . wear a cup.
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    Richgj3's Avatar
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    There is a very good paint shop at my airport. THEY NEVER SAND METAL. Stripping is done chemically and everything is washed with water then pad washed with solvent and scotch brite. Very labor intensive.

    I know of a 421 that the owner decided to save some money by having his auto body friends do it. That one has a future as pots and pans. They ruined it by sanding the rivet heads off. It doesn’t take much.

    The guys at my friends shop go around every rivet with the solvent and scotch brite. His paint jobs win awards and last forever.
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    RaisedByWolves's Avatar
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    Friend bought a polished 120. It has been that way for so long the rivet heads were flattened and the corrugated skin was cracking from being polished so much.


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    stewartb's Avatar
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    My 180 was the first plane Willard Sawyer, an Alaskan legend, painted solo after firing his entire crew. The plane sat outside for 18 years before moving indoors a handful of doors down from Willard’s old paint shop. If I keep the top surfaces clean of birch sap and dust it looks like it was painted yesterday. I have a pic from the day it rolled out. I should do another in the same place 22 years later. It would do Willard proud. Good painters are priceless. Sandpaper? Not a chance.
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    Charlie Longley's Avatar
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    There was a King Air a few years ago that got totaled from inexperienced paint strippers sanding rivet heads off. I’ve never heard of a reputable shop using sandpaper to strip paint on riveted aluminum.

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    Steve Pierce's Avatar
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    From a Cessna 210 pre-buy years ago Cessna said no dimple not legal. Always wonder how much of the skin they sanded off when some of those skins are .020" thick.
    Steve Pierce

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    Ran a paint shop for about 6 years, had to reshoot all the universal head rivets on a Bonanza wing when a new guy got over zealous with a DA! Like Steve said, if you don’t see the dimple, not airworthy!

    Likely if the shop is sanding off heads, they probably aren’t balancing controls either. There are no shortcuts when doing a strip and paint!


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    stewartb's Avatar
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    What does a pro aircraft refinisher use for stripper? Willard gave me about half a gallon of his so I could strip skis and it worked WAY better than anything else I’ve ever tried. I’d like to have some of that stuff for small projects.
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    Not sure what’s in use now, bet we used a product called Sea to Sky as I recall. Used hand pump weed sprayers to spray it on and water to flush it off. Reverse osmosis water system to recycle the water.


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    gdafoe's Avatar
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    I understand what they use is part of the problem. The EPA outlawed the good effective strippers now it takes considerable more hand work. Way more labor intensive than it was a few years ago.
    Gerald
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    Crash, Jr.'s Avatar
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    Up here Finishmaster brings in 55 gallon drums of "the good stuff" and sells it to shops. As far as I know you can't buy the good paint stripper off the shelf any more.
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    Thirty years ago I got some paint remover from “Linda the Stripper” who painted airplanes at Kalispell City Airport. She said it was acid based. Took tar off road pavers! Great stuff, which means you probably can’t get it anymore. She stripped my Travel Air fuselage with probably no more than a brush and a power washer. Sigh…..
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    How is paint removed from fabric planes like the SuperCub before repaint? Seems like sanding would not be an option.

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    WhiskeyMike's Avatar
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    Now that you mention it, I've seen rib-stiching sanded and made useless.
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    Depending on paint/fabric conditions. I have seen it taken off down to fabric with small spatula, dull knife, air nozzle. By the end of the day trying to completely strip and repair fuselage fabric it is sometimes better to recover. Other times MEK is used to take off damaged layers, Dope just needs to be cleaned and light scuff with green stuff new coat will melt in. I have never seen someone try to strip a full wing. DENNY
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  17. #17
    Charlie Longley's Avatar
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    Quote Originally Posted by Tennessee View Post
    How is paint removed from fabric planes like the SuperCub before repaint? Seems like sanding would not be an option.
    You typically don’t take paint off a fabric covered airplane for repainting. Just scuff sand being careful of rib stitches and screws then repaint.
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  18. #18

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    Quote Originally Posted by Charlie Longley View Post
    You typically don’t take paint off a fabric covered airplane for repainting. Just scuff sand being careful of rib stitches and screws then repaint.
    Got it. About how much weight does a plane gain with an extra coat of paint on top?

  19. #19
    supercrow's Avatar
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    Quote Originally Posted by Tennessee View Post
    Got it. About how much weight does a plane gain with an extra coat of paint on top?
    I did a -12 a while back and did all top surfaces and the sides of the fuselage. Used Aerothane with a dust coat and then a light coverage. Just under 10lbs and got the owner a few more years. Amazing how much more flexible the fabric felt after the coating. Paint was thinned about 40% because I wasn't looking for a show finish; just trying to buy some time on the 20plus yr. old fabric. Obviously not as shiny as some like, but will do the intended job. (I don't try for a super gloss finish even on new fabric. Don't want that much paint on it) Nearly 2 yrs later this 12 still looks and feels very decent.
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    gdafoe's Avatar
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    I thought since some of you would be interested in the results of some research, for future reference, I am posting the following.From Textron Aviation.

    12/06/2021 10:29 AM CT - Engineering has provided the following guidance for a MS20470AD fasteners diameter as specified below. The minimum acceptable head height for an MS20470AD3 rivet is 0.035 in. The minimum acceptable head height for an MS20470AD4 rivet is 0.044 in. The minimum acceptable head height for an MS20470AD5 rivet is 0.055 in. If the head marking is visible the fastener is acceptable. If the head marking is not visible measure the remaining head height. If the measured height is larger than or equal to the value shown above the fastener is acceptable, otherwise it must be replaced. [Ronnau, Thomas]

    Also from an aircraft structural consulting firm. FAA Structural Engineering and FAA-DER. The following are calculated minimum head hight values required to produce the tension load for each rivet size according to Bruhn (structural analysts reference author).
    Minimum Head Height for 3/32 0.027, 4/32 0.038, 5/32 0.046.

    I almost forgot to mention. If a Cherry rivet is sanded at all it must be replaced.
    Last edited by gdafoe; 12-17-2021 at 04:05 PM.
    Gerald
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  21. #21
    skywagon8a's Avatar
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    That's interesting, Bruhn has been considered the go to source for these matters for decades. Now Textron's engineers disagree.
    N1PA
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  22. #22

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    I think some specific though is a practical thing here. First question I would ask is what is the purpose of the suspect rivet(s) eg structural/none structural, what would be the failure mode, how many rivets affected out of the total in that part of the structure, monocoque, semi-monocoque, sub structure etc.

    Do we really care if the dimple is gone from a rivet on a boot cowl for example, or a fillet fairing. Non-structural applications both.

    I know that Bombardier, brand new Challenger to be polished. First step was to sand the entirety of the surfaces to be polished, to remove the alclad. There's no question the flush rivet heads got sanded too.

    All this to say that I don't think there are absolutes when I comes to things like this, and good to see some cooler heads at textron (frankly the last place I would have expected them!)
    S
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