Crayon resist takes on many forms, but the most success I have experienced is with either oil pastels or primary crayons. Oil pastels, of course, have oil in them, which is superior in both looks and resistance to water or paint. Primary crayons, or the large crayons used with young students, are the best crayons to use for resist paintings because the kids can press harder to give a thicker coat of wax to the drawings. The key with both approaches is coloring hard and giving the drawing a thick, even coating of crayon. I don't usually have the kids saturate their drawings when they paint, however. I have had more satisfying results simply instructing the students to paint the uncolored portions of the drawing. The "resist" part happens when they slop color onto the crayoned part of the image. I have also noticed that the type of paper used plays an important role in a crayon resist painting. Hard surfaced
papers such as tag board "float" color more easily and the results of the resist tend to look better. Softer papers or those without much sizing tend to absorb color unevenly and will actually leak color under the crayon, giving it a dingy look and texture. As a side issue, I have noticed that metallic-colored crayons resist better than regular colors, most likely because they contain softer waxes. Hope this helps.