For most of the time that I've used an airbrush I've opted to use water, window cleaner, alcohol or other similar household things to thin my airbrush paints. When Games Workshop first released their Lahmian Medium I started experimenting with using that as an airbrush thinner and was pretty happy with the overall results. Of course, the real downside to using GW's medium is that it's pretty expensive if you're going to use it a lot. Add to that the fact that if you try to use any of their shades over an airbrushed coat of paint that uses this medium can create problems... well, I knew it was time to experiment with something else.
That brings us to Liquitex's Acrylic Airbrush Medium . I bought a bottle of this as my next experiment because I've been using some of their other mediums for a while and this seemed like a natural next step. So far, so good.
The consistency of this medium is like that of thin milk (and it even has a milky coloration). When mixed with paint it thins it nicely and it helps the paint maintain its cohesion no matter how little of the paint you decide to add. This means that you can use it to create very thin color layers and not worry about the spray just beading up on your model like you might when you use water. The medium says that it is not matte or gloss, but I have found that it will tend to give flat paints a little more of a semi-gloss finish. I don't think that it's something that the medium adds, but because the surface becomes so smooth that it can't help but be glossier.
Interestingly, paint thinned with the airbrush medium can still be brush painted to good effect. After mixing up a batch of red for the airbrush I had a bit left over in my thinning cup. Since I needed to go over some areas to bring color up a little more I used a brush and my leftovers. The results were surprising. Obviously, since the paint had been thinned substantially the coverage wasn't as dense as the unthinned paint - but you could slowly build up the color by layering these thinned coats and get a nice, smooth finish. Pigment dispersion is very uniform so you don't get much in the way of streaks or brush strokes when brushing the paint in this way. I haven't yet tried creating washes with this medium (since the Lahmian Medium works so well for that purpose) but I will definitely be experimenting with that in the future.
If there's a downside to using this medium it's in the bottle. It comes in large quantities and the cap/dispenser isn't well suited to doling out small amounts of medium. This has meant that there tends to be a little puddle of medium beneath my bottle when I'm using it a lot. I might need to find a small dispenser bottle to transfer small amounts to for day-to-day use.