why does paint shot through airbrush feel like sandpaper?

A thought here: You’ve mentioned the rough surface seems to happen in areas with negative shapes (I take that to mean concave, or recessed areas, such as one might find on the rear quarters of say, a ’59 or ’60 Chevy) and around window frames.

It sounds to me as if you might want to experiment (on something that doesn’t matter if you mess it up) with lower air pressures, especially with either acrylics or lacquers, as both of those paints, when kicked up into really noticeable overspray, can dry partially in mid air, before they get deposited on the model car surfaces. I had to learn about this when starting to use automotive lacquers on model car bodies with my airbrush. Here is what I found upon careful study:

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Airbrushes, with high air pressures (and 25psi is pretty high, believe it or not!) put out paint with enough velocity to create a pretty noticeable cloud of overspray, much of which dries in mid air, and at the end of the side of a model car body (or even around the raised detailing around side windows) actually “eddies” in a bit of a swirling cloud, which can settle on the surfaces of say, the rear of the body shell, in much the same fashion as dust collects on the rear of your own car or SUV when you drive down a dusty country road (surely you have experienced that, or have seen cars that have that effect on them!). So two steps here:

First, I would not assume that any brand or make of paint sold as “air brush ready” is in fact truly thinned enough. What works for one person may well NOT work for someone else, but the technique I will suggest does work, I’ve taught several modelers to do this: For airbrushing, lacquers should be as thin at least, as 2% milk. This is an “eyeball” thing-think of how 2% milk flows off the inside of your glass, it “sheets” on the glass,but very thinly, very translucent. This viscosity works very well with lacquers (waterborne acrylics behave much differently of course). Even with decanted rattle can paints, they may well be a bit too thick for getting a nice airbrushed finish-I find myself often adding a bit of lacquer thinner (again by eyeball!) to get that “2% milk consistency. Think THIN here.

Second, work with only the minimum air pressure needed to get the paint to flow through your airbrush. Most all airbrushes are made for the art trade, and none of the airbrush manufacturers ever really seem to take into account model car builders. So, think of your airbrush as a miniature spray gun, which in fact it is, out of which you want a miniature paint job. I always start by cutting down the air pressure delivered at my airbrush, to the point that it won’t spray even the thinnest of paints. I then hold the trigger down, and open up the material control (which admits paint into the air stream) a little bit, then slowly increase the air pressure until the airbrush just begins to spray. Once I have that airpressure adjustment (and this is best done by eyeball, not necessarily by looking at a pressure gauge) I simply open up the airbrush itself to get a usable spray pattern (about 1/2 inch an inch out in front of the airbrush itself). Now I have a workable spray pattern at that distance, which is far closer than most everyone reads in the instructions as being the proper distance for spraying. Again, miniaturizing the use of the airbrush (think about how far away a production spray gun gets held from the surface of a real car at your local body shop-it’s actually pretty far away,but in miniature, learn to work much closer in than you have been. Now by moving your airbrush along the sides of your model body shell, you won’t be seeing nearly as much overspray in the air around your work, and very little of it will settle around the corners either on the front or rear of your body shell. With concave areas, spray those first, then work outward from them, that will give you the coverage and smooth surface you want there. Think SOFT and CLOSE here.

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That’s my motto in painting a model car body: TSC, or Thin, Soft and Close.

I’d suggest working with either a junk model body or two, or lacking those, get a package of plastic picnic spoons, and practice the above on those. I think you will see a considerable difference in your outcome.

Art Anderson

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