Priming metal is a pain. Cellulose primers work but they tend to be too thick to allow the detail to show through. The brush you use is toast afterwards too so you don't use a good one - which makes things worse.
Car touch-up primer is good and comes in tiny quantities in a pot complete with a brush, but it's still to thick.
Humbrol have just released a nice acrylic primer - Pot number 1 in the range.
Tested on some Monty's Models whitmetal figures, it works well. The detail is still clear and sharp despite the stuff being applied with a brush. To get this sort of result, I normally spray.
Tests on brass weren't quite so impressive. It's OK and no worse than anything else but the results show brush marks. I suspect some light abrasion and serious de-greasing is required to get best results.
It doesn't smell and best of all, the primer cleans off the brush with water, so you don't have to use something from the "past their best" pot.
Humbrol Acrylic Primer.