PURPOSE: To facilitate the formation of joint portions by affixing color joint tapes for joint pattern to the portions which are to be finished to a joint pattern of wall surface being worked and by removing masking tapes after a spraying agent is applied to the entire surface of wall.

CONSTITUTION: On the surface of synthetic resin film 2 consisting of fluorine film 2a and vinyl chloride film 2b, a masking tape 7 consisting of a foam substrate 4 and released paper 6 is affixed to form a color joint tape 1 for joint pattern. Next, joint tapes 1 are affixed longitudinally and transversely on an inorganic exterior board surface 9 before a spraying agent 10 is applied to the entire surface of exterior board surface 9. After the spraying is completed, only the masking tapes 7 are removed. The remaining synthetic resin film 2 is made the joint portions 11. This method shortens the work period.