I may be able to publish my crudely edited video of the repair, but until then here's what I did.
1. Cut out a patch of canvas big enough to cover the tear.
2. Take the painting out of the frame.
3. Apply the patch over the tear on the back of the canvas, using an appropriate adhesive. I used a medium.
4. Weight down the patch with books, making sure there is aluminum foil or waxed paper between the books and the patch so glue doesn't stick to the books.
5. Once the patch dries, paint over the front of the painting where the tear was. It's just a touch up of the small lines of the tear, but make sure the strokes, color and body of the paint sufficiently cover the tear.
6. Let it dry. You're done. Except for a small patch in the back of the painting, it would be hard to tell you had a tear.
Repairing a tear in a painting
