well, one concern is if the primer is going to be compatible with whatever paint eventually gets on the thing because you haven't decided what the top coat could be. It could be Acryllic Epoxy, Urethane, or Enamel, of all of which I have used successfully on various bike projects and each has its own + and - in use.
I think all primers are fairly compatible, but if you ax the paint shop guys they will promise nothing unless you use what they suggest.
I have had good luck with PPG Prima K36, and SpeedySand. With my own truck I am using ZeroRust which so far has worked very well, even over some surface rust that has just been hit with a wire brush. ZeroRust can be a primer for other paint, but, that paint can't have acetone (I think) in it (according to data sheet). I am just going to clear coat the ZeroRust and be done with it because its not a show truck, but a workhorse.
you can see at:
http://home.attbi.com/~iarnstein/wsb/html/view.cgi-home.html-.html
there are examples of painted bits.
