I'd like to know also.
I live in Florida and have gone through 3 doors from Home depot.
They were perfectly straight at the store and at home. Then they developed bulges and pockets below the surface in my garage.
We have lots of heat and humidity.
The third one I primed and finished with two coats of enamel, Three days later there was a 1/8 inch dip in a section.
MDF sounds like the way to go, but what frame under it will keep it straight???
Does anyone have plans to make aFLAT building board.