Not a whole lot different than painting other things. It's done all the time. Prep is the main thing to consider. If there are lumps of paint missing so that there will be dips in the finished job, sanding may make it much smoother. Clean it of all dust and loose stuff as the hew will just flake off if it put over dirt.
Normally using the same type paint as the existing will get better results. Oil base over oil and latex over latex. do it in several coats rather than one single too thick coat.
Go for it!