Jeff,
A sword striper was exactly what I had in mind when I asked, since it is the deflection of the bristles that is used to obtain pressure on the surface to get a steady line.
Could you please ask your mother how she kept the swordbristles from slipping to one side or the other of the corner; think of it was painting the very edge of a triangular piece and not having the bristles slide down each side. It seems to be to be a very delicate balanceing act, and I was wondering what trick was used to get such nice lines.
Karl