Having rubbed the back of the canvas with coarse glass-paper, coat the material with some strong glue, rub down thoroughly, and press until dry. Failure often occurs through not properly removing the air from between the picture and the millboard. The correct method is to place a square of thick paper over the face of the painting and then expel the air by rubbing, with closed fist, over the whole surface, commencing from the middle and rubbing towards the outside edges. If air gathers under the middle, and it cannot be forced out round the edges on account of the glue having set, prick the blister with a fine needle, and, having let the air escape, rub down well and put a weight on the spot for an hour or two.