Thinning Paint

For acrylics, another thinner I have found to work very well is the UMP Thinner, it comes with a mixing chart for various brands of paints. They also have an airbrush cleaner that is blue in color. They both have a strong smell of alcohol, so I'm sure that is the main component, and the cleaner has a few other additives in it, probably similar to Windex. Again, these are ONLY for acrylics (I've personally tried them with AK, Ammo of Mig, Tamiya and MM) due to the smell of alcohol, I would suggest lots of testing with Vallejo before putting it through an airbrush, you dont want to create that snot-like gunk in your airbrush.

Blacksheep is right about not to use alcohol to thin vallejo paints ...use there own thinners (or) use there model air paints ..no thining needed . i just like to thin tamiyas paint (acrylics) with alcohol ...pete 

It sounds wierd,but when thinning Tamiya acrylics,you can use their Lacquer Thinner or their X-20A.They both work well.Some swear by using the Lacquer,I tried it and it works well enough.

For enamels,l like using Testors Universal Thinner #8824

If you ever want to use Vallejo paints when airbrushing, whatever you do,  do not, I repeat, DO NOT use isopropyl alcohol as thinner!!!!

Rangatron. .I use 99%Isopropyl alcohol. To thin tamiya acrylics. .it's cheaper ..I use turpentine for model master enamel paints ..it's also cheaper then there airbrush thinner  (same stuff)..I use lacquer thinners for Mr. Color paints cuss there lacquer basied. 

For airbrushing I'd use Tamiya's thinner (not the lacquer thinner)

I'm assuming you're talking about Testors enamels, I'd use their airbrushing brand thinner. Or mineral spirits/turpentine.