I have rusty filler rings (filler cap rings, where the fuel caps go). What can I do? New ones are well over $1,000 per pair, are hard to find, and a bear to change.
(Search strings: filler neck repair, fuel filler repair, cap ring repair)
Editor:
Run the tank dry, or drain it. Stuff large intact rags into the opening (no small pieces or frayed items), Use Wet-Or-Dry silicon carbide paper to polish the surface smooth. Since the rings are steel, you can initially use a rotary wire brush, but I advise against it due to sparking. For the same reason, I advise doing this the hard way, by hand, starting out with coarse paper in the 160 range, to remove metal. Graduate up to 400 or so, to get a smooth (but not polished) finish. Then clean thoroughly, and apply some premium two-part epoxy primer. I still prefer the oil-based type for this, but it is getting hard to find. Whatever epoxy you use, don’t get it from Pep Boys or Lowes. Get some premium 2-part product from Aircraft Spruce or a local auto-body repair supply store.
If this is all done properly, the epoxy will be permanent; it is tough as nails. It’s a lot of hard hand work that my forearms can hardly do anymore, but at the cost of $1,000 or more for parts, and probably half that again for A&P labor to change the rings, your work and $50 in supplies is much less expensive.
Regardless of whether you repair or replace, make certain you start putting a squirt of ACF50 or Corrosion X at the outer ring and center post of the installed caps. It will prevent ring corrosion and cap center post corrosion (and water intrusion), especially if the plane is tied down outside. If it is outside, replace the cap outer O-rings annually, and the inner ring every three years (if the post is corrosion-free and kept lubed). If the plane is kept under cover, you can probably make it two years for the outer rings and 5-6 years for the inner ones. Note that if cracked outer rings are left on the caps, the fissures hold the water than causes the ring corrosion. Corrosion on the steel center post on the cap is what tears up the center ring. It is sad how much expensive repair is needed on our planes, just due to the absence of inexpensive minor maintenance.