in this way even if the bristles 4 come into contact with a water paint hardening of the paint can be delayed moreover since the surface of the bristles is smoothed as stated above the water paint stuck to the bristles can be efficiently removed by washing them with water or the like therefore the root portion 4 a of the bristles 4 can be prevented from becoming stiff and the tips 4 b of the bristles 4 from expanding the alkalinity of the washing liquid may be either weak or strong it needs only to be alkaline at all though use of a washing liquid of strong alkalinity may adversely affect the resilience of the bristles therefore in this embodiment the bristles are washed with a weak alkaline washing liquid of 8 to 10 in ph which is further diluted with water further the washed bristles need only to be non acid they may be either neutral or alkaline in addition since in this embodiment the washing clears the surface of the bristles of fine dirt and fat stuck thereto as described above the water paint does not stick to any dirt if the water paint sticks to dirt and hardens as it is the bristles 4 may become stiff however if the bristle surface is fully cleared of dirt in advance as in this embodiment no water paint will stick to and stiffen the bristles 4 therefore the problems of.