Never remove a wet painting from its stretcher bars. The stretcher bars are keeping the canvas from shrinking. The painting is apt to generate enormous tension in the wet canvas—somewhat less so with salt or brackish water—as the fibers swell with the water. If the tacks or staples seem to be pulling, add additional staples to the tacking margin or reverse.
If the stretcher buckles or even breaks under the tension of the soaked canvas, this must be dealt with promptly. If the painting is warped (two diagonal corners up and two down) try to shore up the stretcher. Heavy wood or metal plates can be screwed through onto the reverse of the stretcher. A 2×4 or other dimensional lumber piece can be added to the sides or reverse to strengthen the stretcher. Be very careful to not let the screws being used pierce the stretcher and pass into the painting. Best practice is to hold a small metal plate between the painting and stretcher so if the screw goes through the stretcher, it hits the plate rather than the canvas; then you can just back the screw out to a safe depth.
On larger paintings or paintings with very narrow or flimsy stretchers it may be necessary to add cross-bars or cross-braces to keep the longer members of the stretcher from bowing inwards under the enormous tension in the canvas.
There are two options: If the stretcher hasn’t bowed but you are concerned that it might, simply screw a cross-bar onto the reverse of the stretcher, securing it with screws through the cross-bar and into the stretcher. Again, be very careful about the depth of the screws and not driving them into the painting itself.
If the stretcher has already bowed and you have the materials, equipment, and time, you can make a cross-bar. Cut a board no thicker than the existing stretcher to the inner dimension of stretcher before it bowed. You can get this dimension by measuring the inner dimension at the parallel edges. Ideally, cut curves on two ends of the bar diagonally across from each other. Place the new cross-bar member into the space between the two bowed stretcher bars but don’t let the cross bar touch the reverse of the canvas. Because of the bowing, the cross-bar will not fit, so place it on a diagonal with the curved cuts on the ends against the bowed stretcher bars. Slowly, perhaps over hours, twist the cross-bar into a successively more and more vertical orientation. The new cross-bar will need to be secured to the stretcher after each adjustment. A small block of wood can be screwed into both the cross-bar and stretcher on each end of the cross-bar.
If a stretcher breaks, try to work the stretcher back into place. Metal plates or bars can be attached to each side of the break with screws. If necessary, the stretcher can be pulled back into position with clamps compressing the caved-in stretcher to an auxiliary bar along the entire edge.
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