I recently bought a jar of Miniature Paints' Black Primer. It didn't work on bare metal figures. Then, in the latest issue of WSS (Wargames Soldiers and Strategy) there was an article on using dip to paint up 28mm figures quickly. It suggested white spray undercoat to start with but, where armour is concerned, overcoat with black before using a metal paint. I duly sprayed up 16 Swiss pikemen, then used the Black Primer on their armour and chainmail. The result was really pleasing, as this primer is semi-translucent. The recesses came out really black but the high points remained a light grey. This may well be the way I prep all my 10mm figures in future so that the detail stands out for painting.
DP
I quite like white primer, usually with a brown wash. Really lifts the colours of the figures, though I still use black undercoat for armoured figures.
I usually use blacK Gesso as a primer, that is an acrylic resin in water-based emulsion.
I´m very pleased with the result, after dry, the paint adheres easily.
I use white (spray) primer for all my minis, especially 10mm, the figures are so small I find a black primer makes the whole army too dark. If I need to reduce the colours' brightness then a wash with Dervlan Mud, Griffon Sepia or Black usually does the trick