Much of my underpainting is done with cheap Chinese bristle brushes that I get at the local hardware store. There’s a certain safety in using such tools because I can be very rough and splashy with them and not have to worry about ruining an expensive brush in the process.
After dipping the brush into some color, I often test the flow and pigment load on a piece of scrap or palette paper before committing it to the actual board or canvas. In this case the shape the brush left on the scrap paper intrigued me, so I set it aside to play with later while I went ahead with my initial task—washing in rough background shapes on a comp. Later, while waiting for the other work to set, I went into this shape and added some details and highlights to bring out the form.