Someone around here probably has some experience with this, but I'll hazard a guess. I'm assuming you could sew it up somehow, presuming you've got the materials and skills to do something like that. But if it was me, and I really didn't want to redo the painting (which is always an option), I'd take the canvas off the stretchers and mount it on a board, thus sealing up the tear.
Bummer to hear about the painting, man. Got any pictures of it? Or the tear?
Holy crap, I'd do as Noah suggests. I don't think you could get a seemless (no pun intendted) job with sewing it back, OR you could try to glue a piece of canvas behind it, but I'm not sure how it would end....or what you could use. But seriously, how big is the tear?is it just like a poked hole?
For the sake of professionalism, I would just start over. You will probably save more time and money in the long run.
However, if you do decide to have it repaired, get it done by a professional, as an amateur job will be very evident in the finished product.
I'm sure there are many ways of repairing a torn canvas. Both canvas and acrylic paint are surprizingly resilient. However, the best you can ever hope for is a damaged canvas that has been adequately repaired. If you are dealing with a finished work, then the repair option may be your only choice. Seeing as you are only at the underpainting stage, starting over may be a better option. The time and cost to properly restore a torn canvas MAY be greater than the time and cost to start over on a clean canvas.
It's a tough call!
well i'm working with a deadline and the background was airbrushed. A LOT of work settin up and tearing down the airbrush. plus it's a school assignment and the teacher isn't expecting anything more than crap so i'm not too worried. i'd rather learn on something like this at this stage than to have it happen to something more important. i might even learn something.
What I would do in that case is glue a piece of canvas onto the back of the stretched canvas where it is torn. Use waterproof glue, and when the glue is dry I would moisten the back of the canvas with water. This should tighten the canvas up as the water evaporates. I would also put a few more layers of acrylic on the front to cover and obscure the tear.
WARNING! WARNING! - I never never actually done this, but it is what I would try on a piece of work that was not destined for the Louvre
Or you could poke other holes and go all modern art on them and write a 3 pages essay on why the holes represent modern consumerism.
I'm not sure about the wetting the canvas part, isn't there a chance that it would pull the lips of the tear apart when it dries?
Restoring damaged artwork is a field of expertise in its own right. The risk of doing an amateur repair job is ending up with something worst than when you started. That is why my original suggestion was to start over.