What consistency should they be. I was advisedva milky consistency but they just don't stick in my experience
If you're not already using a proper thinner, try something like Vallejo acrylic thinner (they do two types, you want the clear one), as thinning paints with water tends to lose a lot of the stick, whilst the proper thinners retain the stick.
Cool, the all. What airbrush should I get? Sounds mandatory for a great undercoat
If your budget will stretch for it, something like a Harder and Steenbeck Ultra (their entry level brush at about £60), and a small compressor with a tank (about £60+).
Although you can get sets with a cheap brush or two and a compressor with tank for £80+.
Allow for the fact you will bend the needle* from time to time, especially if you're tired/learning, so make sure you can get spares for whatever brush you go with.
As I said in another thread I spent about £400 on my main airbrush and compressor, but it's probably saved me a small fortune on undercoat, and has saved me a lot of time (which is probably the bigger issue for me).
I'm looking at primarily using the airbrush to do my Kroot hounds when I get around to them again, as last time I tried it was a nightmare with a hand brush (this time I'm going to try and use the air brush for everything bar some highlights, teeth/eyes and a wash)..
*We've bent two in about 30 hours of airbrushing over the last couple of months, mainly I think due to being too tired when cleaning it (a slightly painfull thing given the needles are about a tenner each for the ultra)