One related tip that I got from a painter at Oshkosh:  If you are going to get a pro to paint your airplane, once you get the primer the way you want it and sand it reasonably smooth (by your standards) go fly or take it to the painter.  Don't spend weeks with 200 grit paper trying to get it to paint standard.  The reason is simple: no matter how hard you work at it, the painter is not going to like the result and is going to start by rough sanding all over again.  Whatever rough sanding you did will probably help save him time but any finish sanding is going to disappear with the first pass of the board.  Sounds obvious but it didn't occur to me.  Sanding is not may favorite thing and I spent hours on my back under the airplane trying to get that last scratch out.