treating metal tubing
The homebuilt project that I am building uses a 1 1/4 inch diameter with .120 wall 4130 steel tube for the nose gear strut. What is the best way to treat the inside of the tube to permanently prevent corrosion. The tube will be closed at the bottom but open at the top (inside the passenger compartment).
Is there a specific reason the tube is open at the top? I don't like open tubes because they like to collect stuff.
That said, if the tube must be open, maybe you do the best job possible cleaning the inside of it, hit it with one of the rust inhibitors like "The Must for Rust", let it dry, then pour a little epoxy primer in it, put a temporary plug at the end and give it a good shake to evenly distribute the epoxy. Then pour out any remaining primer.
Alternately, if you're building a 40 year airplane, not a 400 year airplane, you could clean up the inside of the tube and squirt zinc chromate inside it. ZC is an excellent corrosion inhibitor.
I agree with Kyle about the top end of the tube being open; after corrosion-treating the tube ID, if possible I would cover the top end with a plastic plug, cap, or something else suitable. I also like his suggestion for using epoxy primer (I use the Poly-Fiber epoxy primer), but I would take a couple of other steps to create a good paint base, since the inside of the tube is probably dirty and oily:
1) With a rag on a stick, solvent wipe the tube ID as best you can.
2) Remove the mill scale from the tube ID using a one-inch diameter (or less, to fit the tube ID) rubber sanding drum with the shaft extended long enough to reach the bottom of the tube, using a piece of 3/8" diameter x .062" wall diameter steel tube--you'll probably need to drill the tube ID very slightly to produce an interference fit to the shaft of the sanding drum; hammer the sanding drum shaft into the tube
3) Chuck the tube/drum assembly into a drill motor and thoroughly sand the nose strut tube ID from top to bottom.
Another alternative would be to lightly abrasive-blast the nose strut tube ID, if you have the equipment available.
After that, I would clean the nose strut tube ID, solvent wipe it again, and treat it with a phosphoric acid paint-prep product available at your local auto store. After creating that well-prepared paint base, I would then apply the epoxy primer, probably pouring in a moderate amount of catalyzed primer, capping the top end, sloshing it around to cover all the interior surfaces, then turning the tube upside down, allowing the excess to drain for maybe a half hour.
Hope this helps!