Either irregular paint flow or too watery paint when airbrushing

Thanks for the replies and suggestions everyone. I've tried several thinners now (Denatured alcohol, alcohol, windshield washer fluid and also just Vallejo Airbrush cleaner). I haven't tried vallejo thinner yet.

Denatured alcohol worked pretty OK, although I found out that the normal Vallejo Airbrush cleaner worked pretty good. Using a thinned ratio of 1:2 (paint: airbrush cleaner) I'm able to brush thin lines of almost pencil size at about 15 psi.

I had a weird one yesterday- same symptoms. I kept thinning, until I know the paint was far thinner than I normally use.l  Then, I happened to try some paint I had mixed up for the project, but a different color.  That worked fine!  I finally decided I had bad paint (it was part new stuff, mixed with another color that had been in my bench for awhile).  I went out and bought new paint, closer color, straight from bottle, and it worked fine. It is unusual to get bad paint, but it does happen. I especially suspect the part of my original mix that was old. It seemed to pour and mix okay, but it apparently wasn't any good.

I use an H&S Evolution Silverline (same nozzle/needle as the Ultra) and, like DoogsATX I find I need to thin Model Air.  Model Air sprays fine 'out of the bottle' with my Iwata Revolution but it has a .5mm nozzle. May be me!  Anyway, I agree with what Doogs says:

DoogsATX

Second, thinning with water makes the paints spray like, well, water. They bead, they run, they separate.

The key is to find a thinner medium that will work with the paint, but that will "cook off" quickly enough to lay it down without running.

My personal favorite is a solution of Future and Windex. The Future will thin the paint, but gives it some "body" when you spray it so it doesn't run everywhere or bead like crazy. And the ammonia in the Windex thins both down quite nicely and evaporates quickly. This mix also gives you a nice semi-gloss finish. 

Future, with or without Windex is a nice thinner for many acrylics but test compatability first. If you don't like the semi-gloss, use Airbrush Medium to thin.

I've had the same issues with Vallejo Model Air (and Model Color and Lifecolor, actually). They're twofold:

First, the paints are too thick to put through a smaller needle/nozzle combo. Yes I know there are people out there who can just dump the Model Air in and it works perfectly, but something about my setup makes that not the case. Could be because I work out in the garage, or the airbrush I use or whatnot.

Second, thinning with water makes the paints spray like, well, water. They bead, they run, they separate.

The key is to find a thinner medium that will work with the paint, but that will "cook off" quickly enough to lay it down without running.

My personal favorite is a solution of Future and Windex. The Future will thin the paint, but gives it some "body" when you spray it so it doesn't run everywhere or bead like crazy. And the ammonia in the Windex thins both down quite nicely and evaporates quickly. This mix also gives you a nice semi-gloss finish. 

Some recent results with Vallejo + Future/Windex on my KV-2 WIP:

Thanks for the replies this far, so it seems to be an issue of the right thickness.

@Nathan T: I managed to spray an acrylic paint which is not designed for scale models already quite fine (at about 29 psi). So I know at what thickness to aim at I guess. Also I have some old Revell paints (14 ml cups, oil based) which I could use for comparison.

Try using Vallejo's thinner. With that smaller tip size getting the proper thinning ratio will take some experimentation. The Model Air paints may still need some extra thinning. Do you have any other paint brands on hand to compare to?

We all have a nightmare when we start to use an airbrush. I can say that I only use the thinner from the same manufacturer as the paint. For Tamiya I mix it 50/50 and the pressure is about 15-20psi or so. If your using Vallejo Model Air paint it needs no thinnning just use it straight form the bottle.

What I do use to practice on is the inside of cereal boxes. They are a uniform colour similar to a primed surface, hold paint well and cost nothing (once you've eaten the contents). This way you can fine tune your mixtures and get used to the brush.

When you clean the brush make sure there is no residure I had a problem with the cleaner contaminating the paint (it was the Vellejo cleaner) I now run some distilled water through after the cleaner and turn the pressure up higher to make sure its all blown out.

Hope this helps

Phil

have you considered using a thinner rather than water, i tried tamiya and humbrol acrylic paint with water and it was a disaster. moved on to thinners after that and started to enjoy airbrushing.