An in-line process for forming pre-finished siding manufactured from cellular poly vinyl chloride comprising embossing, milling, parting, punching, and coating the poly vinyl chloride such that the resulting siding is suited for use in an environment exposed to a wide range of fluctuating temperatures and moisture. The process accommodates the natural expansion and contraction of cellular poly vinyl chloride by producing a novel milled clapboard configuration that, when used as siding, can be installed to create a historically correct clapboard installation.