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Apart from this, a hydroxyl polyester (hereinafter called Polyester J) having an acid number of 0.3 and a hydroxyl number of 60.4 was prepared from 767 g (6.5 moles) of hexanediol--1, 6, 520 g (5 moles) of neopentyl glycol, 498 g (3 moles) terephthalic acid, 730 g (5 moles) of adipic acid and 196 g (2 moles) of maleic anhydride. 1858 g (2 equivalents) of the polyester was dissolved in 2321 g of styrene containing 1.4 g of hydroquinone, and allowed to react with 333 g (3 equivalents) of isophorone diisocyanate at around 55° C. for three hours. 130 g (1 mole) of hydroxylethyl methacrylate containing 2.3 g of dibutyltin dilaurate were added dropwise to the reaction product while feeding air into the reaction system. The reaction was allowed to continue at 60° C. for further four hours until no NCO remained substantially in the system. The reaction product was then cooled with the addition of 30 g of a 50% styrene solution of Polyester J to obtain a 50% styrene solution of an unsaturated acrylic urethane (hereinafter called Acrylic Urethane K). This urethane was found to have a weight-average extension of molecular chain of 380 A.