Just a word of note. You do not need to sand down to primer coat to repaint. The newer paints adhere well to a nicely prepared "substrate" which includes a smooth sanded paint coat of most paints you will find on a car. The paint used will dictate only rarely, most will adhere as noted above.
If you are using high quality auto paint, their tech sheets will tell you what they will go on over acceptably. As for auto paint versus rattle can.... years ago I did a metal door frame in auto paint (catalyzed acrylic urethane to be exact). The owner later added a new grille with inserts to it....
2 weekends ago I sandblasted it to repaint because the grille portion added on had rusted. The grille cleaned up pretty easily with the sandblasted, remaining paint melted away instantly... the auto paint was damn near impossible to remove (while I didn't try too hard except out of curiosity around some bolts that had rusted- but in those areas I did try.) In short... high priced auto paint is a worthy investment. The newer stuff is TOUGH.
(For those curious the paint was PPG DCC Acrylic Urethane using DX Hardener)
