Regarding permanence, they can be said to be more fragile but just as permanent as oils if properly preserved. Some of the aqueous painting techniques have a considerable degree of material flexibility, but none are so elastic as oils and are therefore not suitable for use on canvas. The principal difference between the handling or brushing out of water paints and that of oil paints is the more limited special brushwork necessary for using water paintings because of their extremely rapid rate of drying as compared with the slow-drying of more plastic oil paints. This narrows the working or manipulative properties of water mediums, and limits the brushwork either to free, fresh stroking or to more complex methods, such as allowing the paint to dry and then going over it with other manipulations. However, watercolor and gouache can be used in blended, graduated, and wash effects much more successfully than tempera or fresco, where handling is in general confined to pencil-like touches applied in single strokes and left alone.
Oil paints are the most versatile of all.
Paints with simple gum or glue binders, including watercolor, gouache, and poster colors, dry by simple evaporation of water, and the dry paint's physical properties and solubility remain unchanged. Drugstores will generally sell it up to about a 5 percent solution, this low concentration being deemed harmless. Another older but effective preservative for gum solutions is beta-naphthol used in small amounts. A 4 percent solution of formaldehyde. Formaldehyde is likely to react erratically with some binders and is better suited to industrial than to amateur use.