A composition according to claim 1, wherein the highly crosslinked coating of ethyl cellulose is obtainable by reacting the previously applied layer of ethyl cellulose with an aldehyde of formula R1 --CHO or OHC--R2 --CHO, wherein R1 is hydrogen, C1 -C6 alkyl or phenyl, and R2 is a direct bond or a --CH2 --n group in which n is an integer from 1 to 4.