Paint Thinning

John,

The device you are looking for is called a viscometer, and there are several different designs. For the paint sprayer you mention, that device works because the paint is relatively thick and is sprayed by mechanical means, not air pressure.

For airbrushing, paint reduction (in viscosity) is much greater. Additionally, the effect of surface tension is correspondingly greater, and that of gravity less, for the small quantities of paint involved.

If you made a simple oriface viscometer (The paint runs through a hole in a container of known size and is timed.) the amount of paint is so small that either it would run out before you could start the timer, or the oriface would have to be so small that it might not run out at all due to surface tension.

That's why airbrushing is as much art as technology. With surprisingly little practice, your eye and brain can learn to judge the viscosity well enough in less time than it would take to make and calibrate a suitable viscometer.

In my opinion you are trying to complicate something that doesn't need to be complicated.  The only reason for thinning paint is so that it will flow properly through the airbrush.  Too thin and you won't get good coverage.  Not thin enough and the airbrush won't pull the paint through properly.  Thin it enough so that it sprays properly.  If it doesn't spray properly add a bit more thinner.  If it doesn't cover properly add a bit more paint.  It doesn't have to be scientific.

You're probably going to find that there is no cut-and-dried rule since it varies from one bottle of paint to the next, but 3:1 is usually in the ballpark for me.

Edit .. Sorry Ross, you posted while I was writing.  Triarius always provides very good advice, so I'd recommend listening to him.

Clash of the Paint Titans!

They're both right, and I've learned a lot from either of them. I usually eyeball thinner to paint, and have learned to adjust pressure and distance to overcome too thick/too thin paint. 

Thanks, Scott. Nothing to apologize for. Your post is a good coda for my academic blather!

Thanks. 

 Your answers all make sense.

John Patterson

Savannah, Georgia

As the others have stated there are no hard-and-fast rules regarding paint/thinner ratios and neither are there any magic tools for measuring proper viscosity other than experience.  A couple of things to look for though, is how the paint sticks to the side of the color cup or mixing bottle after being tilted and how the paint droplets run off the toothpick being used to mix the paint.  I'm not sure that I can easily write down any quantifyable rules as to what properly reduced paint looks like, but as get you gain experience, those are a couple of things to look at to help you get consistent results.

Andy 

I use a real high-tech measurement similar to Andy's......I mix it and touch the mixing stick to the inside of the color cup and if it runs down the side fairly quickly and thin it's ready to go.

I do however adjust if the paint is not covering well or grainy looking from lack of thinning.