I find there are occasions that I like to mask out a subject. For example right now I'm doing swans on a lake. I have masked out the swans by the shore line because I am going to put reflections of the trees in the water and when doing a tree reflection in the water it is so much easier to just mask out the swan and do the reflection. I use masking tape but it would be easier if there were masking fluid I could use with oil paints.T, oils don't really benefit from such. They're so opaque. You can draw yer image & paint up to & past the edges & then jus go over. With WCs the big thing is preserving the paper so it acts as yer white.