The finish washer has an increased bearing surface and the sunken design gives a clean appearance. It's used in applications where the decorative appearance is important.

Gutter/Stove Pipe Screws - Head Type Hex Washer Head Slotted - Finish Zinc Plated with White Painted Head

Flat head types have flat surfaces that taper toward the screw body to allow for countersinking.

Flat head types have flat surfaces that taper toward the screw body to allow for countersinking.

Self Drilling Sheet Metal Screw