Danielle,
I just today received a new book I'd ordered, Transparent Watercolor Wheel by Jim Kosvanec, and was reading about gum arabic. This is what Kosvanec says:
Adding gum arabic to a wash tends to thicken it, making it stay put and not creep away. Another benefit is a slightly longer working time, as the gum arabic keeps the wash from drying as quickly as it ordinarily would. Adding gum arabic to paints also makes colors richer and more luminous. A dark color will appear darker, and yet its transparency is preserved. Still another, and to my mind, essential, benefit of using gum arabic in your glazes or washes is that it greatly facilitates lifting color from the paper; with the addition of gum arabic, color doesn't penetrate the paper's surface as much as it would otherwise...
Don't use a brush to drop in gum arabic...(it) will cause the bristles to dry to the consistency of a brick...To drop in gum arabic which has the consistency of maple syrup, I use a pointed spatula knife, although a dropper would probably be a better choice.