Gum Arabic is a binder, meaning it binds watercolour pigments to the watercolour paper. Watercolour painters use it because it adds gloss and vibrant colours to paintings. Self-mixing gum Arabic in paints is tricky because the ratio depends on the result the painter is looking for. Beginning watercolour painters should use manufactured paints with gum Arabic already in them.
Paint and Gum Arabic Ratio
No absolute rule explains exactly how much gum Arabic to add to watercolour paints. Generally, test different levels of the paint mixture on watercolour paper. Wait for the paint to dry. If the dry paint is brittle, then use less gum Arabic. The layers of paints with extra gum Arabic should be as thin and transparent as normal watercolour paints.
Adding more Gum Arabic
Over-binding is a technique where you add more gum Arabic to the paint mixture. Adding more gum Arabic increases the difficulty of working with the paint layers because the paint will be stickier; however, it will also be more flexible. More gum Arabic gives the paint more body and therefore more range to manipulate it. However, this means that you will have to pay more attention to your painting because the paint is harder to remove, making it harder to cover mistakes.
Fixing Dry Paint
To renew a tube of watercolour paints that has hardened, add a few drops of gum Arabic along with several drops of distilled water. The ratio is not an exact formula, but a 3:1 or 4:1 ratio -- that is, three or four drops of distilled water to one drop of gum Arabic -- should work well.
Storing Gum Arabic
Adding a few drops of clove oil when preparing gum Arabic will help extend the shelf life of your mixture. Try using three drops per litre of water used. Also, store mixed gum Arabic in a refrigerator to prevent mould growth. Making small batches of gum Arabic as opposed to large batches that you will reopen multiple times is preferable.