I've been trying out an aqua board, a clay-coated hardboard panel for watercolourists. It's supposed to absorb watercolours like a fine paper, but this wasn't my experience. It took several layers just to get this much colour because it tends to slide off. And it all comes off with water, so it needs some kind of fixative.
On the other hand, if you want to start over, just scrub the whole thing with a wet rag.
Simplicity is the removal of the useless and the unnecessary--source unknown