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The upper wing skins over the fuel tanks in my 701 are held on with sheet metal screws. The plans call for rivets, which would likely be much stronger and lighter, but I guess the builder thought screws would make the skins easier to remove if he ever had to service the tanks. He thought wrong.
Now I'm trying to figure out how to remove about 250 painted-over phillips head screws that are starting to rust. After fifteen minutes of effort I haven't been able to turn a single one.
Does anyone have any bright ideas? So far the tips I've found online are along the lines of "scrape off the paint and give it a good whack with a hammer".
I've resigned myself to ruining the paint job - are there any paint strippers that are known to be safe on 6061-T6 aluminum?
In the meantime, I'm going to try a heat gun to see if that helps.
As far as the paint gluing the screws in place, one trick is to get a cheap set of hole punches (like those used to punch holes in a leather belt, etc.) and try to find one that's the exact diameter of the screw head and use the punch to cut the paint around the periphery of the screw head being very careful not to nick the aluminum panel. I've also found that sometimes a JIS (Japanese Industrial Standard - has sharper corners on the bit vs a Phillips) screwdriver will grip some Phillips-head screws better and is not as prone to strip them out. If you think the corrosion is making the screw more resistant to remove, then definitely try a penetrating fluid (obviously the paint will have to be cut for it to get to the threads) and let it work for a while before attempting removal.
They actually make JIS screwdrivers that you do whack on the handle and it twists the bit like an impact gun, - they're great to remove screws from the engine on an old Honda motorcycle, but a wing isn't strong enough to resist that kind of impact and you'd wind up ruining the structure of the wing!
If you have to strip the paint, auto body supply shops sell a paint stripper called "Klean Strip Aircraft Ultra Paint Remover." Wear a respirator if you use it, it's very potent!
On my 750 STOL, I guessed that I might someday need to access the fuel senders (which turned out to be true when they both started leaking years later) so I painted my top wing root panel separately and after fueling the aircraft for the first time and letting it sit a day or two to ensure no leaks, I then riveted the panel and touched up the heads of the rivets with paint. Drilling out rivets is relatively easy and apparently much easier than removing corroded screws! The panel came off easily and since it was painted separately, there was no paint "gluing" it down and no paint disturbed in the removal and reinstallation.
Thanks, John, those are great tips. I hadn't heard of JIS before - I'll see if that shape grabs any better.
Your idea of painting the tank covers separately is genius.
Drilling out rivets is pretty easy, so if anyone else has screws in their wings, replace them with rivets before it's too late!
I would get one or two screws out nicely and look very carefully at the hole size and condition of the aluminum underneath. As you mentioned rusting screws it seems very likely that you will find corrosion from dissimilar metals under the screw head. If you are fortunate enough to avoid that, it sounds like the holes may well be oversize for riveting back together…. In either case you are stuck replacing a larger section of material, so getting the rest of it off neatly may not really matter…..
Just my 2 cents
If Matt lucked-up and the corrosion and/or hole enlargement/distortion wasn't very much, perhaps the rivet holes could be upsized such as going from an A4 to an A5? However, as in all things concerning modifying major structures, a consultation with Zenith would be in order!
That's what I was thinking, but I'll have to cross that bridge when I get to it.
If you stick with screws, spend the extra bucks for stainless steel screws for reassembly. They are much less inclined to rust or glue themselves into place. Also, place a nylon washer under each screwhead - this greatly reduces dissimilar metals corrosion by seperating the aluminum from the stainless steel.
If anyone is considering screws for a new build, I cannot strongly enough recommend stainless screws with nylon washers under the heads. I have been using them for many decades and almost never have a stuck in place screw. I do have the luxury of hangaring the plane(s) most of those years, but some of them lived outside and did just as well in my experience.