I saw Marla at the Wisconsin bee meeting.  She showed that bees will  coat 
rough wood, but not necessarily smooth wood, with propolis.  Mann  Lake, 
Dadant should take note.  Plane the outside of the board to  facilitate paint, 
etc., but leave the backside (inside) rough.
 
I did a quick search - found others have reported the bee response to  
rough wood, over a period of many years.  One of those things that keeps  
getting rediscovered.  Marla's contribution is the apparent benefit of bees  using 
propolis to coat, seal, and inhibit microbes.
 
Jerry

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