The first time I sanded and painted my mirrors, but obviously didn't get rid of all the powdery stuff and it was flaking off again within a couple of years.
The second time I painted the mirrors on my 97 was around 15 years ago. I sand blasted them to get all the powdery stuff off. Once that was done I only handled then with clean nitrile gloves on so they didn't even get any oil from my finger prints on them. I used an etching primer before whatever black spray paint I used as a top coat. No issues with the coating since then. They key is getting rid of all the powdery crap.
My mirrors are nice and tight and if I ever want to adjust them it takes two hands as the ball joints are so tight. If the mirrors get knocked all it does is twist the arm in the door mount. Reset that and the mirror is still set as it was.