How does he do it?

John's in the basement, working on his office.  (We are finishing off the basement, a room at a time.  The boys' Man Cave was first.)  JT and Luci are playing down there, running and yelling as kids do.

LOUDLY. 

John just keeps on putting mud on the sheet rock.

How does he stay so calm?

Yes, there is a keyboard and a drum set in John's office.  When the painting starts, the instruments move out (and, presumably, the kids will move, too).