Removing powder paint from threads Bought a aluminum frame with rear suspension, got it sanded and powder paint. While they matched the color perfectly, it's smooth and evenly distributed BUT UNFORTUNATELY totally forgot to cover any threads.

Managed to remove the paint from bottom bracket threads, seat post tube and frame head tube with a copper brush, sandpaper and gently using an awl.

Problem is the #5, #6 threads for equipment and derailleur which is a part of a frame.

Could use a tap on #5 and #6, but the threads aren't damaged so there's no need as they are undamaged.

Any way to remove the paint chemically? Tried a hydrocarbon solvent that I left for 4 hours without slightest effect.
Taps are useful for cleaning threads, and much better than forcing a screw down the hole (though that can sometime be enough).  

Derailleur mounting bolts are an unusual size, but assuming you've got a removable hanger that should be held on by a normal metric screw (M6).  Metric M5 screws are common for bottle cages, mudguards etc.

Normally you'd put a few drops of oil on the tap for actually cutting a thread.  When cleaning a thread, grease can be better as you'll pick up more of the muck with it (and leave the threads nicely greased).  If you've got a full set of taps, a second cut tap or even plug tap is enough for cleaning threads, and easier to start without cross-threading than a taper tap.