Spirit varnishes consist of a solvent and a thermoplastic substance (resin). Thermosetting resin oil varnishes dry by oxidation and polymerization and become difficult to remove in solvents. Up to a point, increasing the molecular weight of a polymer will increase the strength of films and give them greater ability to elongate; as well as, a higher viscosity grade. Low molecular weight resins such as dammar, mastic, and poly(cyclohexanone) based resins are more brittle but penetrate paint more rapidly. Resin AW-2 is more brittle than dammar and mastic. Resin MS-2 is similar to AW-2. Resin MS-2A is a special experimental resin. The following resins are of high molecular weight: poly(n-butylmethacrylate), poly(isobutylmethacrylate), 50/50 copolymer of n-butylmethacrylate, and isobutylmethacrylate. Some of the brand names for varnishes of these resins are: Lucite 44, Crystal Clear 550, Synvar, Acryloid, Lucite 45, and Lucite 46. Acrylatemethacrylate copolymer known as B-72 is sold as a varnish under the trade names of Univar and Krylon Crystal Clear No. 1303. A resin of much higher viscosity is AC-33 or AC-55, and used in Liquitex.