Gesso, an artist's primer for acrylic and oil paints, is a necessity when painting on canvas. The thin, slightly chalky material is usually water-based and dries quickly to a hard, textured surface suitable for painting. You should wear a smock or old clothing when using gesso, because it can be difficult to remove once it dries. If you do get gesso on your clothes, the most important thing is to act quickly -- it can be cleaned, but not if you wait too long.
Rinse the gesso spot under running water as soon as you notice it. If it's very fresh, this may be all it takes to clean it.
Apply a small amount of pumice soap to any remaining gesso spot and gently rub it with a clean sponge. If it's a delicate fabric, test an inconspicuous area first. Rinse thoroughly.
Soak the spot in undiluted white vinegar if the spot persists. Depending on how set the gesso is, it may take a few minutes or it may take an hour or more. As the gesso dissolves, rub the spot lightly with an old toothbrush, then rinse.