Painting joints under darker drybrushed flesh. Help!

I’m struggling at the moment with painting joints. Currently working on a bunch of gaunts with grey/black flesh (abaddon black with drybrushed grey highlighting), and then I’ve moved on to paint the joints in wraithbone with a heavy magos purple wash.

I did it this way instead of painting the joints before the flesh because I was worried about the drybrushed grey over spilling, but trying to precisely paint the joints keeps over spilling onto the flesh and ruining the finish. Fixing up the flesh then leaves me with ugly patches of raw black.

I feel like there must be an easier way to do the joints using a fine brush and relying on my hands to not shake, but I’m not sure what that way is.

Any thoughts??