My 18 month old has drawn lines in crayon on my feature wall (currently painted with suede effect paint). Anyhow, I purchased a match pot to cover the crayon but black keeps coming through and goes all smudgey over the wall. The crayons were red and blue but I think there may be some charcoal or something in the crayons. I have tried spraying the wall with "stain stop" but it still keeps coming through. Any ideas on what I could do. Many thanks.