] A coating solution was prepared by dissolving 100 parts by weight of polyether polyurethane (a polyurethane consisting of diphenylmethane diisocyanate and a polyol, containing from 60% to 65% oxyethylene groups on a weight basis) and 5 parts by weight of a trifunctional tolylene diisocyanate adduct in a mixed solvent consisting of 50 parts by weight of dimethylformamide and 50 parts by weight of xylene (viscosity: 4000 cps at 25° C.).