My Shimano 600 EX long-cage rear derailleur is sticking, I expect it's rusty on the inside. I probably won't be able to find another one.

I've read that people have used oxalic acid or iron out to get rid of rust, but will this work on a derailleur? Is there anything I need to know or be cautious of before I try this?

I don't see why it would cause harm. Just be sure to clean off any residue though after you treat it. Rust might not be the reason why it is sticking though. It could be accumulated gunk, or a faulty spring, or possibly (not sure if you've checked this yet), friction inside the cabling.

Oxalic acid will mess up the aluminum if you soak it for too long. First I'd take it off the bike and give it a thorough cleaning and if it still doesn't feel right and you can see rust inside then go for the oxalic acid.

I am with CampyGuy on this one. Oxalic Acid is strong stuff. You could also damage any chrome that is on the derailure.

Better choice is to wash the derailure to remove any oil or grease and soak in white vinegar.

Be advised that rusty derailures are often teamed up with rusty cables/housings. I have found many times that what I thought was a faulty derailure was actually cause by a bad cable.

Glad I asked you guys! I've already started the long process of lubing those cables, and I'll give the derailleur a good cleaning in vinegar... it is gunky. Thanks!

Susan

Disconnect the old cable from the derailleur and remove the chain, then see if the derailleur pivots freely. It probably just needs lube.

New cables and cable housing are cheap and easy to replace. Good cable cutters do make the job easier, though. Get new crimp ends for the cable, too.

Ya, that is something that some newbies neglect to take care of. Make sure that the cut cable housing ends are filed smooth and clean. It is nearly impossible to cut a cable housing without collapsing it somewhat and getting burr inside the housing. Cable Housings nearly always need to be filed or ground to clean them up.

Could you describe this burr a little better? Since all the cables I've see have already had crimp ends on it, I'm not really envisioning what a burr might look like.

So far, it looks like it just needed cleaning and lubing.

Maybe tomorrow I can cut a cable and take a pic for you. The burr is on the cable housing, not the cable.

You know, the housings are made by a single strand of wire wound spiral into a tube. When you cut the housing/tube, the tube crushes a bit and the edge of the wire folds inside toward the center of the housing/tube. If it is bad enough, you can't get the wire through. Even if you can get the wire through, if the opening isn't cleaned up, it will abraid and rub on the cable.

Hope that helps.

Oh! I understand. Thank you. I saw the word "Burr" and I immediately thought metal.

soak outside in mineral spirits or kerosene and all that gunk will clean off with a little bit of brushing. I use a small bottle brush that doesn't have real stiff bristles to get into the nooks and crannies and then something a little stiffer for the rest of the part. Wear rubber gloves when you handle the stuff.