What methods do you use to gently clean up rusty old tools? Which have you found to be the more successful?
As Gregg33 mentioned above, WD40 and a rag or fine steel wool can be a good way to start. Mineral Spirits and a rag/old toothbrush is usually where I start though. Beyond that, I move to a firmer nylon-bristled brush or brass-bristled brush of differing coarseness, and for harsher than that I move to white vinegar.
For soaking in vinegar, I use undiluted white vinegar (standard 5% w/v grocery store stuff) with salt dissolved until saturated at garage temperature (usually ~90F). From what I understand, the salt here acts as a descaling agent, though I, myself, have not empirically tested its effectiveness against vinegar alone. Toward the end of a vinegar soak, I brush the tool (using one of the aforementioned brushes), while submerged in the vinegar, until I am satisfied with the results. Sometimes, I will decide to leave in vinegar for longer, brushing a few times as I go.
Immediately after removing a tool from vinegar, I drop it into another bath containing washing soda (this I dissolve "by feel" ) and dish soap, dissolved in water for a few minutes, making sure to circulate the water a bit while it is in there. This (base) neutralizes the vinegar (acid) and prevents flash rusting.
I then take the tool from the second bath, rinse quickly but thoroughly under running water, and dry immediately, using WD40 or some substitute to displace any trapped water. Most tools then get a thin later of Johnson's Paste Wax for further protection, and Superlube (or other suitable lube) if it would benefit from it.
I use shoebox-sized plastic containers for this, and usually wait until I have at least a handful of tools to clean up.
For minor surface rust, I sometimes submerge for as little as 1hr. The longest I routinely go is 24 hours (though I've gone as long as 48), and have not seen any pitting or discoloration as a result. That being said, a long soak in vinegar will damage chrome plating/paint/powder if it is already starting to flake; I haven't had issues doing this with chrome-plated tools when the chrome is uninterrupted, though these usually get closer to 1hr of soaking than 24 hrs.
Having said all this, this is obviously not practical/desirable for everyone. I am a DIYer and use my tools one I clean them up. I enjoy the process of bringing them back to life and of using them once I have done so, along with the fact that I am just a small part (so far) of each of my individual tool's storied history. This is why it is worth it to me. Plus, I get some damn good deals on the "rusty pieces of ****" that I tend to buy
