Can you use masking fluid on canvas?
If the canvas is well gessoed, it might work, but it would not be my first choice as a surface. Do not use it on raw canvas or any other fabric for that matter. It gets down into the fibers and simply won't come out. Remember that masking fluid is designed for use with watercolor. If you are careful, you can try it with other water-based media. I don't think I would try it with oils because I think the oil paints would simply cover it up and it would be difficult to remove without damaging the painted area surrounding it. Aside from the fact that it is a temporary coating on your surface, the latex also repels water, so the paint does't stick to it, either.
Hope this helps.
Judy Leasure, TDA