| I haven't primered a bass, but I have had primer on cars before. My advice: You don't actually want primer. What you want for a permenant job is some sort of flat or satin paint, basically an ordinary finish but with a flatening agent in it. Primer never stays looking good, plus, it absorbs moisture. I'm not sure that'd be an issue on a bass, but it certainly can't help things. It'll rust out your car in short order... If you really want to do it right, your best bet is going to be talking to a pro at a paint shop that does a lot of work on rods, customs, choppers and the like.
As far as the effect on your tone, it's sure not going to help. All of these kinds of paints are thick and heavy compared to varnish or even nitro laquer, but if you're playing amplified, hey, it might even help, right? | 
