Casting my vote for Brite Touch general purpose primer. $3.50 at my local auto shop and goes on thin. I also use Krylon primer on occasion, as it tends to be fairly thin and the dark gray is almost indistinguishable from Adeptus Battlegray.
Just watch out. Not all auto primers are created equal. I've got two cans at home that I only use for terrain, as they will obscure details like none other.