I painted over the outline of the horse with a fine liner brush. I used thin paint with Liquin and left it to dry for a couple of days then I applied a thin wash of burnt sienna and ivory black over the top of it. This is a backwards way of painting.
After wiping down the wash with a paper towel the outline has faded considerably in some areas, but is still faintly visible. I now have a good foundation for starting the painting proper and I haven't lost the toning by making corrections to the outline. Yet.