To file or not to file?

[nomad]

I went ahead and files the tip. I tinned the tip with rosin core solder after filling and it worked like it used to for awhile. Now I keep getting this black crud built up on the tip and needed to file again. I am using a wet sponge every time, and a paste flux with zinc chloride along with the rosin core solder. That is the only solder I can find that is small enough. Now I am back to filing the tip to clean the crud off or the rail doesn't heat up.
What am I doing wrong ?

Loren