One thing about painting miniatures; you will not improve much if you are not willing to try out new techniques and with this in mind, I embark of a test of sorts.
Traditionally, I have used flat white spray paint as an undercoat for all my minis. I do not use 'Primer' as it will fill-in detail and in 15mm, this is a problem. I prefer to top-coat with a varnish to give the finished mini's paint job durability.
White undercoating, in theory, gives the mini a brighter overall finish but I wonder if the opposite is true. Will black absorb the dark from the paint and cause the color to appear brighter? One issue I am having with white undercoat is that you have to re-paint all the crevice's and deep shadow areas as any missed areas are glaringly obvious.
Another problem I am having is that the white primer shows through the top coat. While this can be desirable for some things, generally it forces me to do at least two coats of Vallejo to completely cover the undercoat.
I have decided to try two brands of flat black paint to undercoat three minis each and relate the experience. I grabbed six Pionere figures and two types of flat black spray paint. The brands chosen: Krylon and Armorcoat. I found the Armorcoat to be cheaper in price but better misting than the Krylon but I suspect the Krylon will stand up better to the scratch test.
Once undercoated, I left the minis to dry overnight. Another observation, I was able to spot a fine casting line that I would have missed with white undercoating (center figure) - thats a bonus! Also the flat is more of a satin for both brands.