More two cents worth from me and probably way more than anyone wants to hear:
Sometimes when one dissimilar metal is in contact with another, it can cause corrosion. This is a common problem and may be what is occurring.
I first clean the part with a tiny wire wheel on my Dremel tool. I then put painters tape around the threads. I then thoroughly clean the exposed part with good clean paint thinner and then let thoroughly dry. A new condition tooth brush works well. Now you don't even want to touch the exposed part, as skin oils will interfere with paint adhesion. I carefully clamp a vise gripe around the taped thread end, using only the lightest finger touch amount of clamping force so as to not mar the threads. I hold the part using the vise grip and then I spray clear paint onto it. It should be at least 70 degrees warm out for this. I then set the part in a out of the way non-dusty spot (still held by the vise grip) and let it dry for at least 24 hours.
For brass bridges and saddles: I first run the correct size thread tap or chaser into all thread holes. I then thoroughly clean the part with Brasso and 0000 steel wool. The Brasso leaves a residue that needs to be completely removed. Thinners work for this and used thinner is OK. I then polish the part to whatever degree of sheen I desire. More polished takes more work and time. Ultra fine wet sandpaper followed by a cloth hand polish with Brasso or whatever you prefer seems to do the trick. I source clean used Tee shirts from the Goodwill Outlet store and cut them up after washing for the hand polish step. I don't recommend a electric wheel polisher on the small stuff, as they seem to be too powerful and can unevenly remove metal if not careful (or toss parts across the room). I attach screws I have around to the part and handle it by the screws only. You must stop touching it now, as skin oils will interfere with paint adhesion. Once again, you must remove ALL trace of polish, grit etc and let dry at this point before painting. The secret to a good paint job is in the prep.
I then shoot clear paint onto the clean part. Saddles I handle by the screw I have put in while holding the screw by a hemostat while painting (hemostats are generally always available at gun shows). Bridges I put two screws and nuts into and then carefully screw into a piece of clean wood so as to stand proud from the wood a bit, and then paint it and place in an out of the way non-dusty spot for at least 24 hours. Again, it needs to be at least 70 degrees out to paint.
You may need to repeat the thread tap work after painting to remove paint, as you want a nice easy turning fit for all threaded parts.
You can also shoot multiple coats of paint and that is a good way to further protect the shine on the brass. This may require rubbing out with the danger of burn through, so I usually don't bother on small parts.
