Yes, I promise, I’m really going to give you the solution.
So, for all of this fuss about paint or medium acting like “beads on a duck’s back,” it turns out the duck knew why all along. So did Max Doerner. Which is a good thing, because if it had been left to my powers of deduction it would have remained a mystery unsolved.
Why does water run off a Duck’s back? Rumor has it (i.e., Yahoo! Answers) that ducks have something called a “preen gland” that produces a waterproof mixture of waxes and oils.
So guess why your paint or medium (when you “oil in”)–or even varnish–sometimes forms beads on an already-painted surface? Yep, oil.
The solution? I’ll pass this on to Max…
If, because of its oiliness, the ground takes colors poorly, it may be rubbed off with a slice of onion or potato, or better still, with dilute ammonia, also with alcohol.
A few sentences later he says something similar, but even more precise:
One must, before using, rub off such canvases as feel oily with ammonia or alcohol in order to improve the adhesion. Rubbing with a potato or onion only prevents the trickling of the colors and does not improve the adhesion.
So something I hadn’t known the answer to for a long time (though I had suspected and experimented with a potato… with modest results) has apparently been known for a long time, or at least since 1934 when Max Doerner published his book “The Materials of the Artist and Their Use in Painting.”
I should add that I have since tried resolving this problem with alcohol and it works like a charm. Just don’t use too much as it will pull just a little color off the surface.
And I must hand it to Max; he knows his paint… and his painting. The last sentence of the author’s preface reads:
It (the book) is not intended as a course of instruction in painting, because it is no more possible to learn to paint from books than to learn to swim on a sofa.
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