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The "museum" or "gallery" finish for final varnishing refers to the method of applying it. A good brush is required (preferably a 2" badger-hair sash brush) and (minimal) skill using it. Varnish is applied with the painting laid horizontally; when the varnish reaches a certain "tack", it's "laid off" by manipulating the brush (using the very tip), and that results in an "upset" finish that diffuses reflection, unlike the glassy-smooth surface that normally results when varnish "flows out".
The final varnishes of choice used to be damar and mastic. Combining the two, 50/50 or with a reduced portion of mastic (to taste) prrovides a varnish that's very agreeable to apply in this manner. I expect numerous posts will follow, relating the pernicious and inferior qualities of both of these natural resin varnishes. Indeed, the modern synthetic final varnishes currently being offered are no doubt much superior to these obsolete, antiquated materials, if not perfect, and it's best to relegate damar and mastic to the dustbin of history, as curiosities, along with many other materials which have made up the pharmacopiae of oil painting through its 500+ year history.
Whether the handling character of "newfangled" varnishes is conducive to laying off a "gallery sheen", I do not know . . . I haven't gotten around to trying 'em yet.
Applying any non-reflective coating is problematic for the presentation of a painting at its intended full contrast and chroma, since reducing glare (even by reducing gloss by laying off a "gallery sheen") can only be accomplished by breaking up the light reflected from a surface, and all additives, whether wax, silicates or inert ingredients, impart a milky look to dark passages to some extent.
If the new synthetic varnishes are compatible with beeswax , (i.e., if the mfgr. recommends adding it) it's the best choice for making up a "matte" varnish. Experiment to arrive at a mix which reduces glare acceptably with the least amount of wax.
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