Oil Painting - Protection of the Picture

The Back of the Picture
The back of a picture can be damaged by puncture, moisture, or greasy dirt. A sheet of heavy cardboard fastened across the back of the canvas stretchers will give the painting substantial protection. In preference to corrugated cardboard, use a heavy-ply illustration board or process board with a smooth water-resistant finish. Another suitable material for this purpose is Fome-Cor [Monsanto Co., St. Louis, Mo.], a lightweight board made of polystyrene foam sandwiched between paperboard facings. It is available in thicknesses of 1/8, 3/16, and 1/4 inches and is sold in sizes up to 4 by 8 feet. Cut the backing board one inch smaller than the outer dimensions of the unframed painting, so that it fits across the back of the canvas stretcher frame. Attach it to the wooden stretcher strips by means of brass screws, 1/2 inch long, spaced about 8 inches apart. Use countersunk washers to prevent the screws from working through the cardboard. Do not seal the edge of the backing [p. 90] board with tape, because some circulation of air is desirable to prevent mold and moisture from accumulating. For the same reason trim back the corners of the backing board so as to leave a triangular opening between the backing and the stretcher frame at each corner. Such a backing prevents people who are handling the picture from pasting labels directly onto the back of the canvas, writing on the reverse of the fabric with a crayon or marker, [p. 91] or leaning a sharp object, such as the corner of another picture, against the back of the canvas. Damage resulting from mistreatment of the back of a canvas may become evident from the front of the picture only at a later date, but injury both to the ground and to the paint film, may be serious although not initially visible. The protective backing board, applied at little expense and effort, will keep the picture in good condition much longer and may avoid the necessity of some costly repairs. [pp. 90-92]

Canvas Keys
If canvas stretcher keys are to be inserted in the corners of the stretcher strips, tap them in lightly and secure them in position by attaching them with a piece of masking tape to the stretcher strip next to them. This prevents them from working loose and falling into the space between the canvas and the stretcher strip, where they would damage the painting. [p. 90]

[Kay, Reed. The Painter's Guide to Studio Methods and Materials. Englewood Cliffs, NJ: Prentice-Hall, Inc., 1983. pp. 127-129]