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Hi, Raymond ! I can only relate what I have found through experience. As noted above, the varnish of choice may or may not be "ideal", and other materials ultimately may be found to be superior. I like a 60/40 proportion of damar and mastic, both varnishes at a 5# cut (i.e., the resin crystals dissolved in good pure spirits of gum turpentine to the proportion of five pounds to the gallon) before mixing them.
Damar retouch varnish sold in art stores is too light in solids; damar alone "flashes" a bit too quickly to manipulate in this manner. You may encounter some difficulty obtaining Chios Mastic (resin of the pistachio trees peculiar to that Greek island (pistachia lentiscus). Looking for it through ethnic food suppliers is a better bet than art materials, it is used in Greek pastries.
It's more difficult to work very large pieces, (over 3' in one dimension) because the varnish may reach a tackiness conducive to "laying off" where you began, before you've applied varnish to the whole thing, so you need to keep working across, following up as needed.
There is no specific time factor, all depends on temperature and humidity where you are working, and the absorbency of the painting being varnished. Apply the varnish sparingly, working it well into the surface, and . . . just keep brushing . . . as you return to the areas of initial application, eventually you'll note how the brush begins dragging, and this is the time to "lay off" the sheen . . . a light source oblique to the surface of the painting is useful, so you can see "wet" areas, and the results of your "distressing" the finish as it tacks up.
I don't follow a specific pattern for brushing, obviously it's more effective to follow a regular pattern so as not to leave "holidays". Something of a random pattern of overlapping strokes works well for me. Keep the varnish brush perpendicular to the surface, using only the flagged tip of the brush. This is why a badger brush is especially nice to use, and a fair-sized one (2") at that . . . makes it all much easier!
At least in theory, this technique could be employed with any varnish at the point of "tack", but success will depend on each varnish's open working time, how rapidly it reaches that point, and whether it is formulated to flow out rather than "stack". Last, as Dave points out, the "bad press" the Nat'l Gallery varnish got was for using a varnish that would not be readily re-soluble and hence, removable. Don't try anything for a "final varnish" that contains drying oils, or forms an insoluble film. Good luck!
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