This is from Alexander Art's daily email. Personally I've had OK luck with sand paper and odorless mineral spirits. However, this would seem to get you back to an almost blank canvas. When I've used my process, you get a "stained" canvas. But, I'm OK with that. The oil paint covers right over it.
Here is Alexander Art's tips:
One of our recent website visitors asked a question about reusing a canvas on which she had a dried painting. She wanted to know the best way to remove the paint and “save” the canvas. A wet painting is easy to remove. Scrape off the paint with a palette knife, wipe down the canvas and start again. Dried paint is different.
I called Alexander Master Artist, Tom Anderson and asked for his advice. It turns out Tom reused canvases often when he had his studio in California. So he gave me a six-step process for removing the dried paint and preparing the canvas for a new painting. Here they are.
Step 1: On a table, lay the canvas flat and cover the painting with a layer of oil. You can use inexpensive vegetable oil. Pour a small amount of the oil directly onto the canvas. Using a cloth or rag – spread the oil over the entire canvas and let it “sit” overnight. The oil will soak into and soften the hardened paint.
Step 2: With a palette knife, begin scraping with a horizontal motion. Lift the softened paint from the canvas. Be careful not to puncture or tear the canvas. This step will remove most of the heavy paint.
If necessary, repeat steps one and two until you have removed the majority of the paint.
Step 3: Prepare a mixture of 2 parts Soft Scrub; and one part water. With a two-sided non-scratch scrub sponge and using a circular motion rub the mixture onto the canvas. Use the rough side of the sponge. This will help remove paint that may still be in the weave of the canvas.
Step 4: Wash the canvas with clear water and let it dry. You can use a hair dryer (“LOW” heat setting) to help speed drying.
You can also repeat steps three and four if you like.
Step 5: With some fine sandpaper, rub the surface of the canvas to remove the last traces of paint. By this point, the old painting should be nothing more than a blurry image. Be careful not to damage the canvas as you sand the surface.
Step 6: To begin preparing the canvas for your new painting, apply a layer of Magic White™; and let it dry. If you want, you can apply a second layer of Magic White. The old painting should be completely covered at this point.
If you prefer, you can tone the canvas with Burnt Umber or Permanent Violet This creates an under-layer for the new painting. You can even use a black paint, like our Artist Black Medium™.