To file or not to file?

[ezdays]

That "black crud" is oxidation. Normally, if a tip is tinned properly, it will wipe off with the wet sponge. If it doesn't, that means that the solder is not adhering to the tip. My thought is that you need to replace the tip and don't file it this time. Just make sure that when you put the solder to it, the solder will flow on the tip and even give you a small blob of solder at the end of the tip. Wipe it on the sponge and it should look really shiny. You can then touch the tip to the work and then put the solder on sparingly. If the iron becomes too hot, it will also cause oxidation on it. In any event, you cannot tin through the oxidation and removing it with a file is futile as you are finding out. Just replace the tip, or if the iron is a cheap one, get a new iron, but perhaps one with a changeable tip.