Often theaters will skin the floor of the stage with Masonite (or hardboard).  It protects the permanent stage floor and provides a smooth surface that can be painted.  It’s also a great sounding surface for tap dancers.  When I was in high school, our stage was skinned with Masonite that we kept painted flat black between shows.  We pre-drilled our holes and meticulously screwed down the boards with a gap between that we spaced using washers.  It didn’t matter how meticulous we were though, it would always end up bubbling and warping especially when it was humid.

When I was in college I learned that all that bubbling and warping could have been greatly minimized if we had simply painted the back surface (and maybe the edges) of the Masonite before installing it.

Why is that? 

Well, Masonite is made by taking wet wood fiber and compressing it under just the right conditions to make it chemically bond to itself.  Unlike a lot of it’s wood-pulp sheet good friends, Masonite doesn’t have any resin or additional binders in it.  That means it is MUCH more likely to absorb ambient moisture causing it to swell and bubble.  If you put a coat of paint on the back, you seal the surface so that moisture doesn’t have as much of a way to get in.  I usually paint the edges when I paint the back.  Then you paint the top and the whole surface is sealed.  Sometimes there is still a little swelling, but it’s NOTHING like what I remember from high school.