I used to use hobby primer. But now I use much more economical automotive spray primer (from my local hardware store).
I prime in white. Many here talk about mixing paints to lighten them for the "scale effect". Well, try a white primer -- you may find this does just what you want, and it's much easier than mixing paints.
Now, as to my pile of pewter, well I may not quite be at rct's level, but I too have a VERY considerable pile. And as BurWolf, I often get about half-way into a painting project and stall out. Sometimes it's just boredom with the subject (the first tank looks good, and I feel all motivated. The second batch of 15 ... not so much). Sometimes it is for want of some ridiculously little issue. Like, right now I am in maybe my 2nd year of trying to finish a single company of US Infantry. I can't find the stupid sandbags I made 5 years ago! I tried making some more, but I didn't make enough (and they aren't quite as small as I want). There's only 2 stands I want to put sandbags on. But GD I can't go on to the next project leaving these two stinking stands half-done, because then I know I will NEVER finish them!
Sigh.
It's actually some kind of sickness. Adiction. Disease. I don't know of any hobby shops that still stock GHQ in my area, but even when I had one just a few (as in 10? 15?) years back, their stock was SMALLER than my stock is right now! And yet, I WANT MORE!
Seriously, I think my partner would kill me if I bought more than I did on Black Friday. She knows how many other minis I have to paint in the house but this was the first GHQ purchase in almost 3 years. It's been too long. I'll quit buying other stuff to focus here instead.