The first thing you must do to repair an oil painting canvas tear is to find a piece of spare canvas that matches the original. If you cannot find a suitable one, you can cut a piece of canvas from a different painting to use as a patch. This patch should be large enough to cover the damaged part of the painting, and allow for 3cm of overlap so that the glue can adhere to the undamaged side. Then, apply PVA glue to one side of the patch.
Before you apply glue, remove the painting from its frame. Carefully lift the canvas from its frame using a spatula to smooth out the tear. Then, apply the repair fabric to the affected area with acid-free glue. Make sure not to over-apply glue because it may squeeze the edges of the canvas, which will make the repair look unprofessional. After applying the repair fabric, cover the entire piece with a sheet of cardboard or a thin plastic credit card.
After you have repaired the canvas tear, you should try to smooth it out again. To do this, you need to cut a piece of canvas that is wider than the tear. Always use lighter colors for this purpose. You should also use acid-free glue and acrylic-gesso to fix the canvas. Do not apply too much glue as this may leave a stain on the painting. When applying the repair, keep in mind that a professional-looking repair can only be done once it has been cleaned and dried properly.