Depends how steady your hand is and what the frame is made of.
I did mine with a very sharp stanley knife blade carefully (and thet bit needs to be emphasised) running round the threads. I only had a few small patches to do so it was fine. Would have taken an age otherwise.
You need something that is harder that is going to remove the paint but not damage the thread. On an alu frame this is harder than steel.
Wire brush would work too althought they are a bit more likely to damage the rest of the paint unless you have a suitably small one. Use wire brushes to clean up threads all the time at work and it is ok on steel.
be really careful with an old bolt or bb as if you cross thread due to the paint you will knacker it. Do it by hand only and take your time. It does work though.