One problem I've had with using masking tape or airbrush mask film to mask off areas of canvas when painting is that the paint bleeds under the mask. This leads to unsightly blobs and runs along the edge of the masked area, and just looks ugly.
The solution is to paint the unmasked area in the same colour as the area under the mask first. So the same colour bleeds into the same colour and seals the edge of the mask. You may need to do apply two coats of the same colour for thin paint.
You can then paint over this, and the new colour doesn't bleed. This does build up the paint fairly high with a fairly sharp border, but that can be an interesting effect in itself, and explains what I've seen on lots of contemporary painting. :-)
