I have had this issue on a couple of Rivs, in place of a tap I used a well
worn M-5 bolt so I knew it wouldnt bind up. I just ran it through a few
times on each of them, it wasnt 100% better but def made it usable. I can
tell you the brazeons got super hot running the "tap" but it did seem to
work.
The "right way" is to carefully run the correct size tap through the
paint-clogged threads. The tap should go through fairly easily if it is the
correct size (3mm or 4mm?). Borrow them if you can, but taps of this size
aren't outrageously expensive, and good to have in the tool kit.
Good luck,