I respect this is an old thread but would like to chime in. Experience; 20 years plastic welding, fabrication of small parts to repair all sorts of items for people, refinishing rigid and flexible plastic, Thermoplastic and Thermosetting polymers.
Since the question was regarding the frame of a firearm, might I suggest the following additions:
1- Checking the fitment of the parts that go back on once you have painted it? See how much more "coating material" you can build up before parts stop lining up correctly.
2- Clean the surface with degreaser or even dish detergeant to remove the surface contaminants and soil.
3- Using a white cloth and a Pre Solvent listed as safe or the type of plastic substrate you are working with. ie; 3M Prep Solvent or other body shop plastic prep-solvent. A product by the name of Polycracker is an excellent cleaner and degreaser.
4- Rinse with hot water then dry.
5- Wipe using a white lint free cloth and a Final Prep or even Isoprypol Alcohol. This will remove the surfactent left behind by soaps and or degreasers.
Remember, when you wipe the surface, you want to go one direction, flip the cloth to a clean spot then wipe another section. Wiping back and forth just moves the contamination around on the surface.
6- The 400 grit should be fine but test in a small area. Some plastic will fur up on you if you use too low "abrasive grit" paper. Generally, I would use 600 grit but again the member that mentioned 400 grit is right on too.
7- Blow off the sanding dust "small project, canned air works well" then perform another final wipe down using the IP Alcohol or Final Prep.
8- Bull Dog is a must as the member pointed out. Use an airbrush, spray gun even a Preval set-up from the hardware store will work. He is right, thin coat, let dry fully, apply another thin coat.
9- Check the fit of the components going back onto the frame. If you have room, spray a coat of primer. If you do use primer, might I suggest using 600 or 800 grit then wet sanding to make it smooth but still abraided enough for the paint top-coat to grab on?
10- Follow the paint of your choice instructions.
Hope this will help a member with their project one day.