It depends on how the frame is built. Obviously, the easiest way is to pull the glass out and then you can be as messy as you want with just the frame!

Recently though I painted a large floor mirror for our master bedroom. The glass was built into the frame (like it is in most heavy pieces), so I carefully made a border where the glass meets the frame with 2″ thick blue painters tape. That way I could really coat the frame well and cover all the old finish.

All that said, paint comes off glass really, really easily, especially AFTER it’s dried. A little windex helps the stubborn spots too.

Hope that helps!
Jenny