My old atelier teacher would have said this is barbaric. But there it is: a small press with the pad of the thumb into the darkest corner, near the edge, where nothing matters. If it sticks, I wait. If the ridge holds its shape and I can't dent it, I go in again.

The reason is simple. I lost a whole passage in a hand once because I trusted the back of a knuckle and it was tacky underneath. Twenty minutes of work pulled up on the rag. The thumb has more nerve in it than anything else on my hand and I trust it.

The bad part is the corner of every canvas I finish has a little flat thumbprint in the varnish of the paint, if you know where to look. My daughter found one last month on the portrait I finished in March and asked what happened. I told her it's the signature nobody sees.

Do you all feel the surface, or am I the only one walking around with a permanently stained thumb?