The mushroom shape can be a great help to a vertical route being just slopey enough to suck the body into the wall, but gritty enough to hold the fingers on. It is also used in some overhangs for experienced boulderers. Using clay, grab the wire brush off the bar-b-q and lightly tap the top until it is saturated with holes. Another alternative is to use the course drywall sandpaper, but it just doesn't seem as effective for this hold.