Many moons ago, brush on primer was in white. I bugged Reaper about it on their forums and other people did as well and hell, black brush on primer. I still have several bottles. See, I live in Chicago and well, this cursed land and humidity go hand in hand. Brush on is always a sure bet.
Still, other companies have come a long way. Vallejo for instance, now has several brush on primers. Games Workshop has a new paint line and their new primer is a black brush on primer, Imperial Primer. When Hobbytown had that sale, I picked up one of those as well as the texture.
So how does it work? I took another old figure I've had just sitting around for like a thousand years. In this instance, one of the Reven for the Reaper Game Warlords.
Some people might talk about smell. My nose is weird. Come around with me with perfume or cats and I'll be out of there. Primer, sealants and other chemicals of that nature don't bother me. Might be my long exposure to them. (Look for post on lung cancer ins 2022 or so) but I can't comment on it because I don't notice it.
Overall not a huge amount of difference and if you're a fan of Games Workshop this isn't going to stop you from using it.
Biggest difference though? Look at the size of the bottles in that first picture. Quite a difference. Now if you will, imagine the price difference. It's a 200 ml bottle. I don't see a quick price anywhere so the 60ml bottle is like $7.00. So that's like $1.20 per 10 ml? And the larger bottle is less expensive per ml I believe. The Games Workshop one is something like $3.70 for 12 ml. Which is under $3.70 per ml but not much eh?
I'll post up pics of the orc when I finish painting him (whenever that is) and see if the different sided primers had any real effect so to speak.