If you are using prepared Gum Arabic a plasticizer will already have been added as the film would be very brittle otherwise. Unfortunately Gum Arabic has very poor resistance to water, pollutants, acid and alkali. It has reasonable resistance to ultraviolet light and fairly good resistance to decay. It will always attract moisture from the air, which leads to cracking. I cannot find anything which will prevent this. A coat of varnish would not be the answer as I believe the pigment/gum layer underneath would move, causing wrinkles or cracking later, I would strongly suggest that you consider using an acrylic binder instead of the Gum Arabic. How you combine the pigment and binder is another matter.

Incidentally, I would be careful when using such pigments as Vandyke Brown as the genuine pigment fades very quickly. It is an organic material similar to peat.

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