I've done this method of painting my nails a few times. I never thought of using it to paint other things, though! That's way cool. 
A few tips and tricks I learned from nails:
- Cold water sets faster than room temp or warmer water. I always let my water rise to at least room temp.
- Distilled water will work better than tap water, but either can be used!
- To mark off places you don't want the polish to go, use cloth medical tape. It will stick very nicely, without leaving a residue when you pull it off.
- Use a toothpick around the edges of whatever you're painting before pulling it out, to make sure that extra polish doesn't come with.
- Make sure to let the paint set (not dry!) COMPLETELY before pulling off tape and/or sealing it! Nail polish can take 24 hours to set properly, so be wary. That applies to the top coat as well, if you're using clear nail polish to seal! You can also dip the piece in an ice water bath for about 15 seconds to set it faster.
You should be able to use nail polish on metallic surfaces. Worst case, it doesn't look right, and you use nail polish remover to strip it off.
A few tips and tricks I learned from nails:
- Cold water sets faster than room temp or warmer water. I always let my water rise to at least room temp.
- Distilled water will work better than tap water, but either can be used!
- To mark off places you don't want the polish to go, use cloth medical tape. It will stick very nicely, without leaving a residue when you pull it off.
- Use a toothpick around the edges of whatever you're painting before pulling it out, to make sure that extra polish doesn't come with.
- Make sure to let the paint set (not dry!) COMPLETELY before pulling off tape and/or sealing it! Nail polish can take 24 hours to set properly, so be wary. That applies to the top coat as well, if you're using clear nail polish to seal! You can also dip the piece in an ice water bath for about 15 seconds to set it faster.
You should be able to use nail polish on metallic surfaces. Worst case, it doesn't look right, and you use nail polish remover to strip it off.