anything.
it depends if its just for the wall or if its going to be skated.
if im gonna skate it (like the arm board), then i make sure to use stuff (paint, ink, pencil, whatever) that won't be sticky (which that stain that i used on the arm board is still drying and im not sure if it'll be tacky or not after drying, but if so, i'll just buff it till it slides again)
but if im not skating it, then hell I'll use glue, melted wax, house paint, dirt, no holds barred, go wild.