Rusty clutch bolt

Unfortunately when the snowmobile chassis was stored, I left the long clutch bolt sitting in the belly pan of the sled.   Now it's covered with rust.   I have it soaking in PB blaster, and have been periodically taking a scotch brite pad to it.   Some of the rust has come off, but the threads are proving difficult.   I don't want to take a wire brush to it since as I do not want to destroy any of the threads.   I will try a soft bristle brush, maybe an old toothbrush, and see how that works.