Roy asked a question on my YouTube channel. He says: "I have recently discovered gouache and like the process of thick over thin. Some gouache artists warn of “dusting off” when gouache is used thinly. Do you have that problem? And if so, how do you avoid it? I have not noticed it occurring so far."
Roy, there have only been a couple of times that I've had the problem of pigment coming off. For me it happened when I used gouache thinly over a casein or acrylic underpainting where the underpainting was too thick and had a smooth surface. As with oils, thin over thick can sometimes be a problem.
Apparently the problem would have happened because there just wasn't enough roughness in the underpainting for the thin film of gouache to adhere to, and not enough chemical adhesion. Gouache is just comprised of pigment loosely held together with a binder. The glue-like binder is gum arabic (an edible sap from the acacia tree), which has a much weaker emulsion strength than acrylic or oil.
If a wash is watered down too much when you put it on, it can result in an underbound film emulsion, which is subject to rubbing off, like pastel or chalk. If you're ever afraid of that happening, you can add a small amount of acrylic matte medium to your paint, and that will strengthen the emulsion.
Even if you use gouache in the normal way, the final surface will be fragile, and it doesn't stand up to much abrasion. (It's also sensitive to hand oils.) That's also true of watercolor pencil and regular pencil, by the way, even if I seal the surface with workable fixative. I've noticed that some colored pencil strokes will rub off and transfer to a facing page. So I try not to paint on two facing pages and try not to handle my sketchbooks too roughly.
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