Re: re using a painted canvas
If you are sanding, it is best to use a good respirator that fits properly.
Some commonly used pigments are quite toxic in dust or spray form, even in very small amounts. The lungs are a particularly dangerous route of entry for these pigments.
If you can avoid sanding, so much the better.
If the painting is bare acrylic, it should be all right just to wipe down the surface with a clean, damp cloth (microfiber cloths clean better than most). Sanding shouldn't be necessary.
You could also use one of the high-adhesive acrylic products, if you are concerned about adhesion.
A raw (unvarnished) acrylic surface should have enough pores and microscopic tooth for good adhesion.
If texture is an issue, you could use something like light modeling paste to even out or alter the surface texture.