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…