I just finished two custom painted dart guns for my boys for Christmas!
What follows is a detailed account of my process for these custom paint jobs. First off is getting ahold of a pair of vanilla nerf negotiators, disassembling them, keeping track of the screw placement, documenting the internal layout, and setting the internals aside.
And then sanding.... lots of sanding... I used 50 grit to knock down unwanted details, 180 to smooth out the sanding marks and a scotch-bright pad over any surface that would be painted.
Once everything was ruffed up it was time to prep the pieces to make them easier to handle and start painting. To prep the pieces I fitted them with a wire using ticky-tack to hold the pieces to the ends of the wire. The wire allows me to manipulate the piece without touching it and allows me to place the piece in my paint block to dry. All the paint up until the touch up and weathering was applied using rattle cans. I began with a base coat of either black or grey applied in several thin coats. After the base coatings I started with the main color for each gun, once again applying the paint in a couple of thin coats. Then I tapped off portions that I wanted to remain the base color and applied the accent colors (usually a metallic.) I got a great tip from Punished Props and started warming my rattle cans in a warm water bath just before I used them. This produced a finer spray, which allows a thiner more even layer to be applied. this works especially well for the metallic paint.
Finally I reassembled the guns, touched up the spray by hand and finally added a healthy dose of weathering. When I was happy with the paint job I applied a couple coats of clear matte spray. This evened out the gloss levels on the piece (the various rattle cans and my own acrylics have different finishes) and will help protect the paint job from the heat of nerf battle.
All thats left is to wrap these bad boys up and put them under the tree. :)