The final coating solution containing the hydrophilic PU prepolymer adduct intermediates is diluted to the desired concentration by means of relatively low boiling solvents such as, for example, pentanes, hexanes, methylene chloride, acetone, methylethyl ketone, methyl tert. butyl ether, or other solvents which can speed up the evaporation after coating of the plasma- or aminosilane-treated substr