How to clean my soldering iron tip or how to determine that it's beyond repair?
(Solid) tip "tinners" are almost always ammonium phosphate with some tin (or tin-lead) bits mixed in. They are moderately aggressive at cleaning oxides off, so shouldn't be used constantly, but only when needed. The best tip tinner is your solder--tips should always be coated with a small amount of solder.
If some of the oxides are just sticking really well, you could try to mildly abrade them on a brass sponge, copper braid, or similar, but you can't be too hard or you will damage the iron plating (good tips are typically copper core, plated with iron, then chromium everywhere but the working area).
Practice good tip care. I use a Hakko iron and tips at work and mine has lasted about a year (moderately light use, maybe 5-6 projects). Put a good amount of solder on the tip when storing it or leaving it idle for any length of time; don't, for instance, wipe it off, put it in the holder, then leave it on over lunch.
Additional guides:
- Hakko Tip Life Checklist (PDF, attachment to above KB article)
- Adafruit Tip Tips (PDF)
Well, the solution to my problem turned out to be both very simple and very unexpected.
I decided to buy a real stand for my soldering iron and the one I got came with its own sponge. It did a great job cleaning the tip and the tip looks like brand new now!
The sponge itself as a spare part is just about 2.5€, so the moral of my story is don't try to save money on the sponge! Buy a high quality sponge and save yourself lots of headache!
Cheap soldering irons (the unregulated kind that run at 10-25W and the power cord goes straight into the handle) have a lot more problems with corrosion than the nicer regulated irons. It doesn't matter too much if it's ugly, as long as you can get a nice shiny bit right at the tip. Dedicated tip cleaning mesh is the best thing I've found for that, but the traditional damp sponge works fairly well. Just try to wipe it off after every 4-5 solder joints.
Don't worry too much if the whole tip isn't nicely covered in solder. You only need the last 5 mm or so - the part that comes in contact with the parts you are soldering.
As a last ditch kind of thing, I've been known to go after a badly corroded tip using a metal file. I'd only do this on a really cheap iron, but I had one where the tip had been eaten away in an irregular pattern and I was able to reshape it and get some more use out of it with the metal file. Do not try this with a fancy iron like a Weller or Hakko that has a ceramic tip.