i havent use Airbrush for some time now (dont have the space & way to messy)
Actuly, gamesworkshop Paint is not maide for airbrush.
-Your tips will try up whit pain all the time
-will be to watery
-will mix badly
what you need is to buy Model car paint (for remote control cars). (acrelic)
now carfull, that stuff can smell bad!!! (well mine did, but damm good paint)
but you will get the best of effects
i recomend airbrush for a few things only:
-making the first cover of a large model
-Jewels (when you get good at it)
-First basic color (like a blood angel)
to get beter at your airbrush , get a large peace of something Flat & practice a few days.
Thats all i can say for now... ask your questions
i did do Airbrush for 6 years so, im not a total idiot in it.