The process is shown in the diagram to the left.
The pre-finishing process even allows for control of exactly how much coating goes on each board, shingle or piece of siding.
A simple explanation of the process is that the piece has the desired coating flooded on and then brushed and back brushed through a series of hydraulically operated rollers.
The pieces are then racked in a drying room until cured. Finally they are re-bundled and wrapped for delivery to your job site ready for installation.