A piston rod assembly comprising a piston rod for pumping paint and a packing retainer, the piston rod comprising: an elongated cylindrical stem, the cylindrical stem having an outer diameter that is uniform along most or all of a length of the cylindrical stem; and a piston rod cap connected to the cylindrical stem, the piston rod cap being cylindrical and the piston rod cap comprising an annular top portion having an outer diameter that is larger than the outer diameter of the cylindrical stem; an annular bottom portion having an outer diameter that is larger than the outer diameter of the cylindrical stem; and an exterior recess located between the annular top and bottom portions, the recess having an outer diameter that is smaller than the outer diameters of the annular top and bottom portions, wherein the packing retainer surrounds a portion of the cylindrical stem such that the cylindrical stem extends out of opposite sides of the packing retainer and the piston rod cap is outside of the packing retainer, the packing retainer holding one or more packing seal rings around and in contact with the cylindrical stem, characterized in that the cylindrical stem and the piston rod cap are formed from metal, and in that the recess has a depth in the range of 0.254 to 3.18 mm (0.010 to 0.125 inches), and the recess has a height in the range of 0.762 to 9.53 mm (0.030 to 0.375 inches). an elongated cylindrical stem, the cylindrical stem having an outer diameter that is uniform along most or all of a length of the cylindrical stem; and a piston rod cap connected to the cylindrical stem, the piston rod cap being cylindrical and the piston rod cap comprising an annular top portion having an outer diameter that is larger than the outer diameter of the cylindrical stem; an annular bottom portion having an outer diameter that is larger than the outer diameter of the cylindrical stem; and an exterior recess located between the annular top and bottom portions, the recess having an outer diameter that is smaller than the outer diameters of the annular top and bottom portions, wherein the packing retainer surrounds a portion of the cylindrical stem such that the cylindrical stem extends out of opposite sides of the packing retainer and the piston rod cap is outside of the packing retainer, the packing retainer holding one or more packing seal rings around and in contact with the cylindrical stem, an annular top portion having an outer diameter that is larger than the outer diameter of the cylindrical stem; an annular bottom portion having an outer diameter that is larger than the outer diameter of the cylindrical stem; and an exterior recess located between the annular top and bottom portions, the recess having an outer diameter that is smaller than the outer diameters of the annular top and bottom portions, wherein the packing retainer surrounds a portion of the cylindrical stem such that the cylindrical stem extends out of opposite sides of the packing retainer and the piston rod cap is outside of the packing retainer, the packing retainer holding one or more packing seal rings around and in contact with the cylindrical stem, characterized in that the cylindrical stem and the piston rod cap are formed from metal, and in that the recess has a depth in the range of 0.254 to 3.18 mm (0.010 to 0.125 inches), and the recess has a height in the range of 0.762 to 9.53 mm (0.030 to 0.375 inches).