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Michelle,
It is important to get some authoritaive advice on this subject in as much as most of what has been offered so far, while well intentioned, is opinion.
I suggest Ralph Mayer's "The Artist's handbook of Materials and Techniques" be referenced to find your answer. It is authorative, technically sound and thoroughly researched. Every artist should have a copy in their studio.
For instance, copal is a varnish not a drying medium, and copal has its own special archival concerns. Mayer's research will set matters straight for you.
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