I use PPG Deltron urethane high build primer. Primer alone works great. Dark grey and wet sands/buffs out to a mirror finish. Unless your plug is perfect, I'd be afraid that a low build epoxy primer would be difficult to buff out without multiple applications. I do however use white epoxy primer to product primed in the mold parts. I'd agree that dark colors seem to make it easier to detect flaws. For that reason, I also add black pigment to the tool coat of the mold. Dark color also makes it easier to see if the layup is sitting correctly.

Pics of a plug I did a few years ago can be found here.

http://www.rcuniverse.com/forum/m_40...tm.htm#4070588

Working on a plug now for a turbine, plan on using the same technique as it worked well for me previously.