The fact that oil or wax and water don't mix can be used when painting to mask out areas to retain the white of the paper or the color beneath, as well as to create interesting textures. Quite simply, you draw with the wax on your paper, then wash over it with a water-based paint. Where the wax is, the water in the paint is repelled and so the paint either runs off it or collects in little drops on it.
What's the Difference Between Using Wax Resist and Masking Fluid?
Ordinary masking fluid you rub off once your painting is dry; wax gets left on the paper (you also get permanent masking fluid, which is designed to be left on the paper). Masking fluid is a total block - when you rub it off you're left with a solid area of pure white paper - whereas with wax it depends on how solidly or evenly you apply it.
What Types of Wax Can be Used?
Any, though the results will vary depending on how oily or waxy something is, how absorbent or textured the paper is you are using, and how thick your paint is. The cheapest form of wax is probably a white candle. Next wax crayons, then oil pastels. Don't limit yourself to white wax crayons or candles only, remember it's the wax not the color that repels the water. Do tests on small sheets and keep a record. Experiment with broad sweeps of wax and sharpening the point for precise lines.
Help, I Can't See Where I've Put White Wax
If you hold your sheet of paper to a light, you'll be able to see the wax shining in the light. Being systematic in how you apply white wax, working from one side of the painting to the other, will help you remember where you've already put it.
Can the Wax Be Put Over Paint?
Yes, wax can be used to mask a painted area but must be applied only when the paint is dry. If it's still wet, the wax won't 'take'. In the two examples below, the one on the left had wax applied to dried green paint and was then washed over in orange; in the one on the right, wax was applied to dried red paint, then a stronger red wash applied on top. In both you can see how the original color shows through where the wax was applied and how texture is created by the paint collecting in a few droplets on the wax.