Gum Arabic vs Masking Fluid in water colour painting
This Masterclass demonstrates the difference between masking with Masking Fluid and masking with Gum Arabic. You can use either method to repel the paint from the paper, keeping areas of your water colour painting clean and white. Both methods are effective, but will give different results: The Masking fluid gives a clean, hard edge to your masked out area, while Gum Arabic gives a softer edge.
From the Winsor & Newton Site: