If using a coping saw, the blade will be vertical and easy
to judge. However, some of us are accustomed to using power
miter saws to achieve perfect cuts and are unable to confidently
cut directly on a line by hand. Trying to improve the situation
by filing or sanding the end grain usually just creates more
solution is to discard your coping saw, not the cope joint.
Waste away the bulk of the material using a jigsaw or handsaw,
then use a right-angle grinder equipped with an abrasive flap
wheel to accurately cut/sand to the line.
toward the line.
Another effective method is to use a flex shaft equipped with
Kutzall carbide burrs powered by a Foredom or Dremel®
tool. Slight back cutting, so that only the front edge touches,
guarantees a closed joint; too much will yield a fragile edge.