Back when I started airbrushing I used VMA paints+Vallejo AB thinner, then I managed to hunt up a local store that stocked the Liquitex AB medium and used it instead of the Vallejo thinner. Never had any real issues with those two in combination with VMA paints.
But that store closed down a while ago and after I depleted my stock, I decided to try out two recipes from other guys instead of simply ordering a bottle of Vallejo AB thinner.
Version a): 50% distilled water, 25% Liquitex Slow-Dri and 25% Flow Aid... that's the mixture Andrew from Schnauzerface uses. As far as I can remember I went with a bit more water (55%) and only 20% Flow-Aid.
Version b): 50% distilled water, 10% Matte Medium, 30% Slow-Dri and 10% Flow aid.... but I went with 25% SD and 15% F-aid
I skipped a bit on the flow aid in the first recipe, because the label on the Liquitex bottle recommends a 20:1 water:FA ratio and 25% of FA looked like a awful lot to me.
Earlier this week I tried both of DIY thinners with some colors from the Reaper Master Series through on some 28mm Norman kite shields. Instead of drying in a slightly prolonged period of time, the color coat turned out to be splotchy and took ages to dry.
<= took a picture halfway during my painting session... RMS templar blue thinned 1:1 or less with the DIY mix.
I then wiped off the still wet paint after a minute or two, cleaned my AB and tried again. Same paint, but thinned with a few drops of distilled water. This time I got a smooth and even coat, despite using the same thinning ratio, distance and pressure. No blotches, dried between the quick passes...
My guess would be that the additive ratio is too high, but I'm not 100% sure if it is something that can be fixed. The next things I want to try is a) thinning down a portion of the already mixed paints 1:1 with more distilled water and b) mix some distilled water with the flow-aid according to the label on the bottle (20:1 parts) and then use that mixture with 25% Drying retarder. Since the label on that bottle says "don't add more than 25% to the volume", it shouldn't breach the producers recommendations as long as I don't thin down my paint too much.
Has anyone suffered similar mishaps and if you did, what was your solution?