The water-based medium for the water-based resin used in the present invention may be water itself, while it may contain an alcohol solvent such as ethyl alcohol, isopropyl alcohol, n-propyl alcohol or n-butanol or a water-miscible organic solvent such as mono- or dialkyl ether of ethylene glycol or diethylene glycol in an amount of up to 50% by weight based on the water based medium.