Well, I'm not a paint expert, but in most cases the surfaces should be scuffed up to allow the paint to better adhere. My guess would be to use a steel wool. It would use enough fine scratches without scaring the surface like sandpaper. If you used sandpaper I would go with a 1000 grit or something like that. Then I would clean them with a paint surface prep cleaner. Finally spray with a self etching primer, followed by a coat of paint.
I would think that is all that would be needed. You just want to make sure that what ever you use to prep the sills that it doesn't pit or scratch so deep that primer and paint won't fill in to get that nice clean look.
Anyone else here have a better plan of attack.
In lieu of steel wool, try red scotchbrite pads. They won't leave the residue that steel wool does.
To strip them, try a little laquer thinner on a rag, follow with the scotch pad.