not intending to start another "copal medium" onslaught of posts here, Dana...but that is actually the original traditional intended use of that medium.
As a natural siccative, it attaches to the pigment and dries uniformly throughout. It has been a medium of choice for glazing. Some spread the medium over the area or work without pigment, then paint some paint directly in and scumble. Let dry, and repeat the process again and again. Others mix paint into and apply. Copal used this way would be dry in a couple days.
I'm sure there are other good sources of its use, and Ron Garrett on his site explains the techniques. Studio Products site is another good one on techniques and mediums.
I'm the one that doesn't use the stuff traditionally, because I paint more impastoish and alla prima. I found out more recently though that many of the American early landscape painters used "oil of copal"...the Cape Cod, Hudson River....etc; painters.