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Any oil painting should be varnished if you care about how long it will last.
The dry spot phenomenon known as "sinking-in" is usually due to the use of burnt umber, more than anything else, and/or to the practice of thinning the paint with a solvent. When burnt umber is used in too high a concentration, it will defy varnish, as it is highly absorbent. The varnish will be drawn into it and disappear. I seldom use burnt umber any more, for that reason. Raw umber has this tendency, too, but to a lesser degree.
Using glossy mediums can also cause irregularities in surface gloss. I've found the best way to achieve a fairly uniform surface appearance is to not use burnt umber, not thin the paints with solvent or with medium, and instead add only enough linseed or walnut oil to the paint to make it brushable. I add the oil on the palette, using an eyedropper, and mix it in thoroughly with a palette knife. I also avoid the stiffer brands of paint, so I won't have to add so much oil. Try it and see how it works for you.
Virgil Elliott
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