I just sanded the whole thing with 220 grit and used rustoleum 2x paint. It seems to have adhered well.
The Boomco rifle is easy to disassemble, there are only 2 screws that are longer so just note what holes they go in. I didn't paint the buttstock piece or hand pump piece since they were already a nice black. The ammo counter sticker will tear in half if you do take it apart, I replaced mine with a simple static one with blue LED backlights.
I stuffed some LEDs and a audio module in my rifle and now the seams don't line up as nice as before so I may consider using some bondo or other filler eventually, but it isn't very noticeable unless you're close to it.