Copal is a great medium, no other blends as well. Garrett Copal of Clovis, NM is the very best I found. I swear, it makes all other brands pale by comparison.
If you use copal, you must use Garrett Copal.

"Copaiva balsam oil and copal resin both dissolve the lower layer and really slide the paint around... I never liked the brands I tried until I tried Ron Garrett's, copal@3lefties.com
there wasn't enough control. His brand painted beautifully."

These two painting were done with a visiting student, because we painted a RCW color wheel also I wanted to blend the colors with every advantage, she was new to oil paints.

This paintings took 2 days Here is the web page that goes into details.

http://www.realcolorwheel.com/annabouquet.htm