If you have to paint metal parts for your model, I would suggest using 'white pigmented shellac' (not the water based type) as a primer. The trick is to first prepare the surface.
1-Lightly 'rough' up the surface's) with a fine emery cloth.
2- Dip the parts in a cleaner (white vinegar is ok).
3- Rinse with cold water and let dry.
4- Apply a coat of shellac, let dry, then apply your paint.
Try not to touch the surfaces with your fingers after cleaning (use tweezers). I'll mount some of the parts on a wire or toothpick and stick the toothpick into a block of soft balsa wood. The shellac works on just about any metal, and sticks like glue to the surface. You can apply any type of paint on it, acrylic or oil. Clean up is easy, just use ammonia (and hold your nose!).
I use the Zinsser brand called "Bin":
Bin - Shellac , by Zinsser
One quart of these will last for a very long time. I haven't tried the spray can yet, but that would probably be better for larger parts.
If the parts are copper or brass and you're looking to 'blacken' them to give the appearance of gun metal, then try using "Brass Black". We have a link for that here:
Blackening of Metal parts