Shellac can be used over virtually anything. It shines best if over a smooth surface though. Sags and runs as Splint suggested can make the end result look less than perfect.
The rattle can approach has the potential for the best end finish, but using spray paint is a muscle memory kind of exercise, and if you want flawless doing some trial sprays will not hurt in getting you up to speed. I also find one of those sprayer handles allows you to do better. Combo of hand ease, and not having to think about that pinpoint spray button.
Old house paint certainly can be used, but it does come with those brush strokes. If that is your best option look to see if a foam roller can cover the trim on your frames, if so using them will make for a smoother surface.
