Advice for painting cowlings

I'd argue that the yellow color is the main culprit. It's more translucent, allowing tbe underneath color to affect brightness and exaggerate the uneven coverage.

When spraying yellow or red, shoot some white first. The white will cut down on the color coats and give you a more brilliant and clean looking color. Yellow can be challenging and is best sprayed as I described and at 10-12 psi in very light coats, building up the shade as you go. 

bstarr3

Usually thinning about 50% with Windex, or to the 2% milk consistency, works pretty well.

Don't thin with Windex.

If it's your first airbrush job, use the manufacturer's thinner to get a baseline as to how the paint is supposed to behave.

When I have this problem on highly curved surfaces, I really dial the flow back a lot, and get just a thin layer on first.  It definitely does not color much, but it adheres better. I wait a minute or two, then build up a little thicker coat. Then repeat. I find it takes several coats in this situation.

I couldn't agree more with this. Just another unnecessary variable.

When I started back in the hobby, I read of folk using windex as an acrylic airbrush thinner. And I wasted time experimenting with same. I now reserve Windex for cleaning glass and the occasional unfortunate stripping of acrylic paints.

And yes, yellow is a difficult color, lots of good advice above on how to deal with yellow.

Edit: my post makes no sense. I meant to quote Phil's suggestion not to mess with Windex. My bad.

Thanks for all the replies.  I ended up sanding the surface smooth of irregularities and brushpainting another couple of thin coats on to get a decent result.  Unfortunately, as I've mentioned in other threads around here, I have no local hobby shop and the only local art supply store is Michael's.  So my options for acquiring Testor's acrylic thinner are to order it online and wait a week, or make do with what I've got.  

I'm planning to buy some Mission Models paint for my next kit, and will use their thinner and primer.  

A simple weathering technique when airbrushing yellow on prop tips, cowels or other parts of the model like many German WW2 aircraft is instead of first spraying white, spray a very light tan, sand color or a light grey.  

Make this coat uneven.  Spray in a mottle or haphazard pattern so that the color is not an even coverage.  You can even go heavier coverage between panel lines.

When you spray the yellow over this you will get great weathered tones of yellow and the yellow won't be as intense like it can be over white.  Add the yellow slowly in very thin coats.  

Thin the yellow 70/30 thinner to paint so that it takes a bit to build up. Shoot it at 10-12 psi so it does not spray a lot of paint and keep moving as you spray so you don't get runs since the paint is thin.  If you don't like the result just add more thin layers of yellow till it evens out.