When I traveled to Glasgow, Scotland I visited the St. Mungo Museum. I surprisingly came across a world famous Dali painting, Christ of St John of the Cross. Any person who has ever opened an art book would recognize this masterpiece. As I got closer to it I noticed a slight mark running through the painting. It was quite large in fact, around two feet. I was shocked to find out that the mark was an act of vandalism. The painting had been slashed by a student in 1961. He claimed he acted for his religious beliefs but I think the muppet was drunk! The painting had been patched up in house. Ever since then I have always wondered how difficult it would be to repair a painting.

In this lesson we will be repairing a canvas. Although It would be best to get a trained repair conservator. Sometimes thats not an option. They can be a bit pricey and seeing I am writing to mainly artists we will more than likely attempt this with our own work. Leave the damaged Van Gogh's to the professionals please.

The wound we will be looking at is one inch or so.

Step 1 Any frayed ends of the canvas should be neatly removed.

Step 2 You should prepare the canvas by moistening the back of the damaged area. Then use books to flatten the surface. The water will draw tightly the fibers in the canvas. Try not use too much water, dabbing will do. Modern canvas are woven on power looms and have a very tight weave and are prone to shrinking. This will damage the the painted surface as the ground may flake off in time.

Step 3 Like a band-aid we need the patch to be larger than the wound. Feather the edges so that when the patch is added you will not see the edges of the patch from the painted side of the canvas.

Step 4 To attach the patch you will need to pick up some BEVA® adhesive.

BEVA® GEL is an aqueous dispersion of acrylic and EVA resins.  In its pure form, or diluted with water, BEVA® GEL is an excellent water-based adhesive dispersion, easy to apply, with good slip, and adhesion to a very wide range of surfaces.  When partially dried, BEVA® GEL turns into a strong contact adhesive with excellent adhesive to canvas, polyester, and Beva-coated surfaces.  To achieve this, high-molecular polymers were incorporated in its formula, which give it the desired tackiness and retention of solvents.  For these reasons, it dissolves only slowly. 
When fully dried, BEVA® GEL turns into a heat-seal adhesive, with the same low-activation temperature as that of BEVA 371 (60-65°C, 150-160°F).  BEVA® GEL, BEVA® 371, and BEVA® D-8 adhere to each other in every form of application.

REF:www.conservators-products.com/beva_gel.htm

Step 5 With some wounds this may be enough to get by. But with our wound it is still very noticeable. We will need to build the wound up to the the level of the canvas. We will have to use a proprietary, fine surface filler paste incorporating PVA (Polyvinyl acetates, the white glue you used in grade school), which will give it flexibility. This combination will allow you to sculpt to the texture of the canvas.

Step 6 Now the painter can paint over the wound to match the original work.

Note - Even Christ of St John of the Cross had a scar after restoration. Understand that it is extremely difficult to bring the painting back to 100%. No smiles Guaranteed.

Surface Denting
Sometimes when canvases are stacked against one another a canvas can be dented by the corners of other canvases. The cotton in canvas can be unforgiving and it will take your efforts to bring it back to its original tightness. Use the same dabbing method, also called "cupping" described in Step 2. This should bring the painting back to its original state.

Protection
Using backboards are a way of minimizing the tragedies that happen to canvases. If you decide to add a backboard make sure the back of the canvas is clean and dry. You can use a soft clean brush on the end of a vacuum cleaner. Or blow off the dust with an air compressor. The Backboard should be thin plywood or acid free poster board.