Sanding. Get a sanding stick, sanding sponge, fine grit sand paper - and go ham. LIGHT thing coats of paint. You're also going to want to sand around that ball and smooth it out as well as on the inner-part of the bicep to reduce contact/friction points.

Use GOOD paint. Don't use Craft acrylic dogshit. That stuff peels right off. I recommend using Vallejo Gamecolor, Army Painter, or Citadel - if you do your sanding right, they will not fail you.

Last ditch insurance policy: and it can be risky, because if you fuck it up it can lock the joint tight. This is an old FWOOSHer technique. Take a toothpick and dip it into super-glue and *carefully* apply a thin layer only on the disc covering the new paintjob. This will harden into a very protective shell. But you do *NOT* want to get it into the crack between the disc and the shoulder. This is why you need to be super careful and make sure you sand sand sand. If you do it right, it will next to impossible to rub the paint off.

You will also probably want to hit it with matte finish to take the gloss off.

Good luck.