paint thoroughly, as otherwise bubbles of air may be covered, which, coming through the paint, render the coating porous. It is known that the durability of any oil paint depends largely upon the number of particles of pigment upon a given area of surface, and that the more particles of pigment the better the protection to the binder and to the metal underneath; the evil of thinning too much is, therefore, obvious. As to the spraying of paint upon structural steel by means of an air compressor, it is argued that this method conveys air and moisture to the metal and corrodes it; besides, its many other disadvantages have led to its abandonment wherever tried. The adherence of paint will be increased if the metal is moderately heated before it is primed, because if it is warmed by artificial heat the surface will be dry.  "Mill marks,"