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Zachary, without inviting any "wars" over painting mediums, suffice to say that with the exception of properly oiling out sunk-in passages in a thoroughly dry painting, "overcoating" with any painting medium is inadvisable, and not the same thing as applying a "final varnish".
The purpose of varnishing a painting is to provide a protective barrier between the finished painting and environmental elements that will degrade it - dust, dirt, smoke, mild abrasion, etc. The archival intention is to be able to remove this protective coating at some point, along with embedded dirt, thus preserving the painting as it was when finished.
To that end, the traditional (at least for the last 150 years or so) approach has been to apply a soft resin varnish which remains soluble in its parent solvent. Damar is one, mastic is another. As mastic alone is more prone to yellowing or discoloring in time, damar has been preferred.
Final varnish is not diluted as a retouch varnish is, but generally is a saturated solution, usually a 5# cut. (five pounds of resin dissolved in a gallon of turpentine).
Properly varnishing a painting requires some experience and skill; the painting should be laid horizontal on a table, the varnish applied evenly, then manipulated to a desired lustre. I prefer a mix of damar and mastic (about 60/40) because the mixture is far easier to control than damar alone, and easily laid off with the brush to a less than high-gloss finish.
There are now final varnishes on the market compounded entirely of synthetics which promise to be an improvement over the natural resin varnishes. Gamvar is one which comes highly recommended by people I trust., but I haven't gotten around to testing it yet.
A parting thought - a painting should be thoroughly dry before applying the final varnish, and the state of dryness has less to do with specified time lapse than the nature of the paint films and ambient conditions. As a rule of thumb, usually six months to a year, though.
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