Masonite is a manufactured product made from wood that is broken down to its basic fibers and then rearranged to form hard panels.
Masonite boards are 3mm (⅛") thick and can be used as a stretching board for watercolour paper, a painting surface or a drawing board.
To use as a painting surface, very lightly sand the smooth side and prime it with several layers of gesso to create a smooth, rigid painting surface for oils or acrylics.