I don't know if this will help you & I cannot answer about paint brushing. I have however had a dilema something like yours so I used reg. Crayons & colored what I wanted to achieve. Then I heat set them with papertowels underneath and over the colored section. So far so good. There was a time that Crayons were used to color the whole design on blocks. HTH
I don't know why you couldn't paint the area you wanted to paint. But setting it, I don't know how you'd do that unless you also painted it with something to set what you used to paint. I probably just confused you. I'm sorry if I did. :wink: