Liquid frisket has been used for a long time. You can get white and in
For water color, you can apply to the area you do not want to paint for now.
Either you may keep the area blocked with white or you may choose to use
different colors later on, on that area. You can use with Acrylics as well.
It is easily removable and when I say that, you can literary peel it off from
the paper. The brush is supplied for easy application or you might have to use
your own brush and remove it by rinsing off the liquid after your application.
Sometimes you can spatter the liquid and block the area for night sky or lighting.