Unfortunately, I don't have any magic tricks for getting nice, smooth, even white markings when painting. As far as I know, there isn't a trick to it beyond lots and lots of very thin layers to eliminate brushstrokes. I do like to seal the entire horse very well with Dull Cote, then airbrush in a thin (not fully opaque) layer of white over the markings, then clean the edges up using a bit of water, airbrush cleaner, or Windex on a q-tip or toothpick, but it's still a tedious process. I always want to rush it and then have to go back and fix things that I messed up in my rushing!
So, I'm sorry this is not a time-saver! It seems like there's no way around it if you want lovely smooth white markings!