Method of painting building boards

A method of painting a building board by a plurality of computer-controlled ink jet painting machines so that colored paint patterns are formed on a surface of the building board, so that when aligned with other building boards, said building boards form a continuous pattern, and each ink jet painting machine including a number of spray nozzles disposed over a conveyor for conveying the building board, so as to cross a direction of conveyance of the conveyor and valves for opening and closing the respective spray nozzles, the ink jet painting machines being supplied with paints of different colors from one another respectively, the method comprising the steps of:

• setting data corresponding to a colored paint pattern to be applied to a peripheral portion of the building board and storing the data in a memory of a computer;
• painting the peripheral portion of the building board by controlling the valves of the ink jet painting machines on the basis of the data stored in the memory so that the colored paint pattern whose data is stored in the memory is applied to the peripheral portion of the building board;
• originating by the computer random data of a random colored paint pattern to be applied to a portion of the building board other than the peripheral portion;
• and
• painting the portion of the building board other than the peripheral portion by controlling the valves of the ink jet painting machines on the basis of the originated random data so that the random colored paint pattern whose data has been originated by the computer is applied to the portion of the building board other than the peripheral portion.