Can one of you airbrush pros help please?

could have been just a bad can of paint. it happens sometimes.

Interesting to hear your experiences Milair.  At least stikpusher finds them good to use!  I had wanted to stick with Humbrol as it was also what I used as a kid and they are readily available here.  Oh well!

Slightly OT, but;

I have used Humbrol enamels both when I was younger & after the 2nd coming & haven't really ever had much issue with them apart from the odd tin being lightly pigmented & thin.

I generally use Tamiya acrylics these days, but needed some colors that weren't in their range (unusual that...), Humbrol were listing the correct acrylics, so I got some to do the job. To say I had a hard time with them would be an understatement - behaving like they were under thinned one minute & over thinned the next & little consistency from a ratio which had just previously worked - I wouldn't rush back for them.

Yes it would appear that Humbrol and your airbrush do not get along.

You can send those poor, unwanted, no longer needed, tins of Humbrol to my loving home where they will be put to good use

Thanks all- I think I may have solved this (fingers crossed).... I turned it up to 20 and put the Humbrol through 1:1 - still the same problem.  I could sometimes pull the trigger all the way back and nothing would come out.  Out of desperation I tried some Revell enamel with Revell thinners that I had and straight away things seemed pretty good.  So there we go, the solution seems to be not to spray Humbrol!

I don't understand the high thinning ratio, but there is another question that comes up.  What did you prime the plastic with.  Sometimes kit mfgs do use a mold release and that needs to be removed, though primer can sometimes eat through that stuff and make life easier.  Some paints just do not work well on unprimed plastic regardless of how clean the surface is.

I doubt raising pressure will help.  15 psi should be enough, most airbrushes will work well even at lower pressure.  Consider the airbrush as a kind of brush- not a high volume spray gun.  Keep airbrush near your surface and spray with a lot of overlap.

Those Tamiya Mixing sticks are great, aren't they? I originally used them for stirring until one day during a demonstration presentation at our AMPS chapter I saw a guy use the little measuring spoon end. After then with a little math in my head and boom... consistently perfect mixtures and no wasted paint or thinner.

Thanks guys - looks like I should try upping the pressure.  I'll do that tonight and try a 1:1 mixture at 20psi.  I'll let you know how it goes.  

Doogs - I mix almost exactly as stikpusher describes, even down to the tamiya metal stir stick (spooky).  I once saw a pro mixing in the airbrush cup so in my beginner's naivety I have tried mixing in the cup once before.  It didn't go well!! won't be doing that again.

If pressure of 20psi is needed to get a good paint flow, how do you get close enough to the model to do finer detail like panel line pre-shading without getting too much turbulence?

Humbrol is my favorite brand of paints to use. But I have never tried their own thinner for airbrushing, or anything else. I prefer to use Testors Model Master Universal Enamel thinner (used to be sold in a black can, now sold in a red can) which seems to give enamel paints more "bite", when they are airbrushed on. All that aside, I stir Humbrol paints thoroughly (opposite of a Bond Martini, stirred not shaken) in the tin first off. Once I feel they are ready I add the amount of paint that I think I will need for the session into my mixing jar. For this process I use those Tamiya metal stir sticks which have a blade on one end for stirring and a little spoon type cup at the other other for transferring the paint. I can count out the drops of paint used exactly and use an eyedropper to transfer the thinner into the jar. Then I add the MM airbrush thinner until I get my "milk like" consistency. Usually this is about 2 parts paint to one part thinner ratio, but occasionally on some of my older tins where the paint has really thickened it is 1/1 parts paint/thinner or greater. With the Testors Airbrush Thinner and Humbrol paints mixed as I described I have NEVER had any clogging or splatter. I do suggest upping your air pressure closer to 20 psi and you should be able to increase your paint to thinner ratio. Your mixtures do sound way too thin and more like a tint than paint.

When I used Humbrol Enamels I usually cut them about 50/50. Something seems a bit of if you need to thin Humbrol that much.

I havn't shot Humbrol in years, but I'm not sure they really like to be thinned that much. It might break the binder.

Also, stupid question, but where are you mixing the paint? If it's in the airbrush cup that may be the problem. I've had enamels fall through the thinner and pool in the needle channel where they really clog up the works and are difficult to get out without breaking everything down.

I don't use Humbrol, but I do use Model Master enamels from time to time...strange that you're having to thin that much. I routinely spray around 15-20. I'd recommend upping the air pressure a slight bit, and double-checking to make sure the paint is well-stirred.

Another trick I've found for when I'm shooting enamel really thin is to cut it with some high-grade lacquer thinner (like Tamiya or Gunze). Makes it a lot less prone to runs and it dries better/faster.