Mucking up the painting up from the start did relax me, I must admit, but since it was on the iPad there was always a thought in the back of my mind that I could undo this. So...not as brave as I could have been.
The video of the process is below. I firstly did a sketch before a 2 hour live model session and scanned it in, Placed it on a layer, and went ahead.
Done with the Brushes App.