was attained only after 1 month from the treatment measurement of air permeability was undertaken with respect to sized paperboards of the present invention prepared in accordance with the procedure of example 1 and also with respect to the comparative sample which was sized with the epoxy modified organopolysiloxane as in example 1 in this case however varied sizing amounts as indicated in table i were employed and the results of the air permeability and the cobb values as determined after 1 3 and 7 days from the treatment are set out in the table as is evident from the data in the table the epoxy modified organopolysiloxane necessitated a sizing amount as much as 300 g or more in order to attain practically suitable cobb values at the sacrifice of air permeability on the contrary the cobb values of the sized paperboards in accordance with the present invention could sufficiently be low even with very small sizing amounts and this was reflected in turn on the much higher air permeability table i______________________________________ sizing air perme amount ability cobb value g 100 cm sup 2 g sec 1 day 3 days 7 days______________________________________present 67 40 lt 0 6 lt 0 3 invention 133 60 lt 0 4 lt 0 2 130 60 1 1 3 0 6 1 2 167 120 1 1 3.