A modern synthetic paint, made with a resin derived from acrylic acid, that combines some of the properties of oils and watercolour. It was the first new painting medium in centuries and has become a serious rival to oil paint. Acrylics are a refined version of paints developed for industrial use and can be applied to almost any surface with a variety of tools (brush, airbrush, knife, sponge, and so on) to create effects ranging from thin washes to rich impasto and with a matt or gloss finish (various additives can be used to modify the appearance). Most acrylic paints are water-based, although some are oil-compatible, using turpentine as a thinner. Thinly applied paint dries in a matter of minutes, thickly applied paint in hours—much quicker than oils. In the 1920s and 1930s the Mexican muralists (particularly Siqueiros) experimented with industrial paint when looking for a durable medium for outdoor use, but acrylic paint, as the term is now understood, first became available to artists in the late 1940s in the USA. Certain American painters discovered that it offered them advantages over oils. Colour Stain painters (see Colour Field Painting) such as Helen Frankenthaler and Morris Louis, for example, found that they could thin the paint so that it flowed over the canvas yet still retained its full brilliance of colour. David Hockney took up acrylic during his first visit to Los Angeles in 1963; he had earlier tried and rejected the medium, but American-manufactured acrylic was at this time far superior to that available in Britain, and he felt that the flat, bold colours helped him to capture the strong Californian light. Hockney used acrylic almost exclusively for his paintings until 1972, when he returned to oils because he now regarded their slow-drying properties as an advantage: ‘you can work for days and keep altering it as well; you can scrape it off if you don't like it. Once acrylic is down you can't get it off.’ In spite of these differences in properties, the finished appearance of an acrylic painting is sometimes more or less indistinguishable from an oil, and some artists have combined the two techniques in the same painting. In addition to being versatile, acrylics are less susceptible to heat and damp than traditional media, although some doubts have been expressed about their permanence.