Jim, I use Vallejo quite a bit and really like it. The best for airbrushing is the ModelAir line,
with the black caps. They are supposed to be able to airbrush right out of the bottle,
but I still thin them just a little with either water or Tamiya X20a thinner. Not much is required.
One little trick, drop a stainless steel nut in the bottle...it helps a lot making sure you have it
all mixed well before using it.
The Model Color line really does need thinning...I think it is basically for figure painting,
but with thinning I airbrushed it with good results. Hope that helps.
Cheers,
Ernie