I'm not sure if you have experience using watercolors, but they move around a LOT, hence the need for masking fluid. The nice thing about oil paint is that it stays exactly where it's put, so there is really no need for masking fluid.
I don't know how hard the stuff would be to remove from the texture of the canvas later. When I painted a lot in watercolors many years ago, it was even hard to get it off the watercolor paper sometimes, if I left it on too long.
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