With all due respect to you , Bob, Masonite is the worst thing on the planet to use inside a car with all the temperature changes they go through. Masonite is an interior product which does not belong outside. Masonite is made from a slurry of sawdust and glue that is steamed and made into panels under very high pressure. Masonite is brittle, and breaks very easily. It is not waterproof, and no amount of sealing will make it that way. Even the oil impregnated stuff will take on water if it comes in contact with water. It also warps very easily. Some older cars had Masonite door panels, but they stopped using it because of it's many failings. Yes, it's cheap, but there's a reason it's cheap. Please do not use Masonite for your interior panels.
No one lives forever, the trick is creating something that will.