Between the foam sometimes being thicker than nominal, and the GG expanding out, usually it was a matter of sanding the foam fair to the wood. Mind you that my "frame" for the walls is only a 2x2 base board and surround around the door. All the rest of my blocking is 1x let in with the router and glued in. I got pretty good at setting the blocking just barely over flush so that it required a minimum of sanding to bring it in. Any trouble areas and dings got filled and faired with light weight "ceiling" spackle.
To tell the truth, since I haven't put the canvas on yet, I can't really say how it will do, but I have gone to great lengths to try and make the walls as flawless as I can so that warty stuff doesn't telegraph thru. Fingers crossed.