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Thread: Fuselage Tubing

  1. #1

    Default Fuselage Tubing

    Well Ive been blasting my fuselage and have found some interesting rust. It appears to me (but I could be wrong) that piper primed there aircraft with surface rust on the metal. While blasting, right at the point where the paint disapears and right before the blasting gives the metal that satin look, I am finding really shiny smooth metal with rust spots. The rust is not deep and goes away instantly and I could swear that rust was there when they painted my fuselage at the factory in 55. I have also found some rust in the tail that has gone through the tubing in a few places but when I opened the area up (fully expecting to find it rusted on the inside), it was new as the day it was born. Also have a couple of places where the tubing has split like water got in there and froze but its not in a low point and there is no way for water to get in there. Mighty strange. Anybody had similar stuff happen? Anything other than the obvious that I should be looking for? Im blasting a section, inspecting, leaving the areas that need repair bare,and spraying Super Koropon on the good stuff before it flash rusts. Its a first for me blasting a fuselage and man is it alot of work!

  2. #2


    Make sure you do the SB for rust around the door frames.

  3. #3
    Gilbert Pierce's Avatar
    Join Date
    Nov 2007
    Germantown, Tennessee 01TN


    Piper originally painted the tubing with left over dope according to Clyde Smith. Rust can start under paint, even epoxy primer when it is not impervious to moisture.

  4. #4

    Join Date
    Dec 2007
    Upstate New York


    PJ; The rust you are finding where you describe probably is NOT "Piper's" rust. A '55 was covered in cotton, and the airplane was probably recovered SEVERAL TIMES before you came along. Piper did their airframes with some really good primers and on top of that some really good dope-proof paint. Contrary to popular belief, they didn't just "squirt some zinc-chromate on there and call it a job". That would have been done in the field, and it turns out it never really was "adequate". Piper apparently knew that to begin with. If the rust is not deep and goes away almost immediately as you blast through, it is relatively "new" rust. If zinc-chromate was ALL the last guy used to prime the then blasted airframe, he screwed up (and a LOT of people DID -and worse, a lot of people STILL DO!). Primers (except for two or more part epoxies) almost universally allow air to pass "through" the primer and access the steel underneath. This does not happen "overnight", but it does happen. Again, I doubt your swearing Piper was the cause of all this is not true.

    I don't fully understand your statement that you have found rust that has gone through the tubing, yet the inside was new as the day it was born. Maybe what you are talking about, is this: First, there CAN BE "inclusions" in any hot-worked steel. It is not unusual for such inclusions to be "raw iron lumps" in the alloyed "steel". These can rust out and leave "pinholes" in the tube, which if well sealed (primed AND painted) will keep moisture out of the tube, and therefore the inside surface will not rust. Still, the inclusion CAN rust (and it does). I have seen sections "in" ("along?") a tube nearly as large as a dime, and when cut out, were the ONLY area in the vicinity that has rusted out (or rusted at all). Inclusions are expected, and the quality control for steel alloys can only do SO MUCH to prevent them. But there IS an "acceptable limit" to how much (many?) inclusions there are in any run of steel. Furthermore, the "advertised strength" of an alloy takes into account an acceptable level of inclusions (that said, NOBODY is particularly happy with pinholes in their spar carrythroughs!!!) So... Not unusual. Good find. Repair as necessary. Secondly, a careless person could have spotted the area with some caustic solution, and never realized it had been done.This can also cause "Mystery Rust Spotting". Don't matter as much when compared to the "first" item as long as you are removing it and there is no disqualifying pitting...good find. Repair as necessary. Some people hate sandblasting as much as I hate stripping paint, and chemically strip their airframes. I do not think this is a good practice, and frown on it.

    Split tubing (of the kind you seem to be describing) is ALWAYS from freeze damage. Because you are seeing it in an area that isn't normally "a low spot" doesn't mean much. I have found such tube splits halfway between the tailpost and the next Station forward, right along the middle of the span. Have a Clipper fuselage right now with one of these splits IN THE UPPER LONGERON at this location! What that is telling you is that there was a SUBSTANTIAL amount of water inside that tube. How it got there doesn't much matter at this point. As a lot of people chemically strip fuselages, they rinse copiously. Rinsing caustic chemicals can supply this water which gets trapped inside a tube. If the fuse was "stood up" at some time during it's storage, there is why your freeze damage is "not where it would be expected". Ya see, strange things can happen. Struts full of water needn't always get swelled up all the way that the water stands inside, but can be found swollen only near the very top of that water...the first area to freeze (just like "on your bridges") will be the area most remote to heat transfer. Bare, hollow steel will get colder faster (what "busting ice" LIKES) than the same cross section of the same steel member that is filled with water! So water in a strut (or a longeron stood upright) will usually freeze at or near the upper level of that store water first, and "hardest" (again, same reasons as why a pond or lake "skims over" first at the surface and may NEVER freeze at depth. The water "below" transfers heat around the whole mass, but the hollow tube focuses it at the edge of the water. "Cold is defined as the absence of heat."

    Bottom line on the tube splits: Good Find. Repair as necessary. Upstate New York sees it's fair share of this type of freeze damage (not to say to the exclusion of other Northernmost Lower 48 States). I gotta pretty good idea that Alaska sees its share, too.

    Yepper...a lot of work, with a standard-type sandblaster that most small shops would have available. I have my fuselages done by a commercial blasting operation by an Airhead that works there, and they are powered by a 40hp screw compressor, and he's wielding a 2" blasting hose. He KNOWS not to cut the 1/4" channel off'n there, and is pretty good at it (and I'm one of about FOUR people allowed in the blasting room that doesn't work there!). Any little pinhole opens right up on one pass. But a good operator that undestands what he is doing with that Ferocious Monster is worth every penny. It ain't cheap, but me, I'd rather pay HIM for the heavy-duty stuff rather than make the investment to to it "right" myself. But I've done my share on blasting ONE fuselage for three days, and that has a lot to do with farming them out!

    Priming as you go is a good practice, but those times I have done short sections at a time with a 3/8 airline, I have used zinc-chromate for short-time protection. Since zinc-chromate actually exchanges zinc molecules for iron ones, you are actually "mini-galvanizing" the bare tube, and that's "good". When all is done, THEN I "go in there" with the GOOD STUFF and seal the fuselage. I'm into two-part epoxy for priming fuselages, and unless somebody else already owns it (and vetoes the extra expense), I usually put Ranthane polyurethane over that. White, so you can spot a crack from six feet!

    You're on the right track. Keep up the good work.

  5. #5


    Where did you get super koropon? Do you have experience working for NASA? My buddy works for them and says I should use it as that is what they used on the shuttle. I've been unable to find it in convenient spray cans.

  6. #6
    rocket's Avatar
    Join Date
    Dec 2010
    5NK in the Bristol Bay, AK


    Koropon is a two component epoxy not available in a rattle can. Your best bet is to ask a reputable shop for a pint of each part; bring two new cans.

    I would recommend any of the two part polymide(sp?) epoxies. Axion used to sell a Boing spec BMS10-11 in a couple of colors. Now PPG sells the same product but in two gallon kits: green or bright yellow. Koropon is an ugly olive drab and it is posable it is not available any more.

    Perhaps you could get together and split a kit with some of the locals in your neck of the woods?.

    I would never never never never use anything else on tubing repairs; Alaskan field tested!

    Remember- "BMS10-11"


  7. #7


    I'm not 100 percent on this, but I don't think super koropon was ever available in convienient spray cans. If you google it, you can find a supplier, its a two part system. But if you read the lable on it, it is intended as an interior primer and not an exterior one, which makes me wonder a bit on it's usefulness on our fuselages and such. There are two part epoxy primers out there that are probably a bit better suited to our applications. (tube and fabric). Something like the Stitts product and then cover that with a good two part top finish like Aero thane. The fuselage would be well protected. Probably much better than with the super koropon. Just my 2 bucks. (inflation)

    Bill Meredith
    Kingsland, Tx

  8. #8

  9. #9
    Steve Pierce's Avatar
    Join Date
    Nov 2007
    Graham, Texas, United States
    Blog Entries


    I used to use the Polyfiber (Stits) two part epoxy. I mixed the green and white half and half and then top coated with White Randthane. Now I use PPG DP48LF epoxy primer. Way easier to apply and can be top coated in a few hours if needed.

  10. #10


    Super Koropon is available in three different colors. I use BAC452, is the (Boeing Aerpsace Corp. (BAC) color. a real pretty green..looks like a granny smith apple color. It is still used by NASA on the space shuttle and Boeing uses it on all there aircraft as most other companies do to. I worked on "heavies" and corporate jets for 20 years and when it came out, thats what everybody started useing. Once cured pure methaline chloride will not strip it, it has to be ground off.

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