With out a base (primer) the wood such as mdf tends to soak in the paint.. I know its a lot more work to paint wood and get it nice, as why I am amazed with steve meades towers with piano black finished paint.
also, if experiancing dimming more when the system is bumping, a second battery will be very helpfull.. but its always good to start with basics, big 3, grounds to bare metal, no loose connections, etc.
Some cars from the factory come with wimpy grounding system/points (like my trailblazer) and will experiance dimming from even using multiple basic switches at the same time. ex) I raise both front windows at same time and my headlights, dash lights and interior lights dimm like a muddah. You can start minimizing the dimm by upgrading grounds and such, big3 w help me a good amount... but a pounding subwoofer never helps the casue but sure does give grins.. lol
I just did my mdf box with rustoleum bed liner (about 9 bucks a can) and did not use any base coat. I used about 2 1/2 cans for full coverage including port. This brand bedliner tends to be good stuff but THIN, which when I was planning on doing mine wanted thinker stuff. Either way I liked the stuff but since I kind of wanted more texture to it, I might stone coat it black a little later..