This is the medium-speed option of the three available catalyst speeds (fast, medium, slow) for use with House of Kolor UC21 Universal Acrylic Urethane Klear. It’s a hardener for UC21 clear, mixed at a 2:1 ratio (clear to catalyst). House of Kolor catalyst is for use with House of Kolor paints only — due to their unique chemistry, House of Kolor Catalyst should never be substituted with other brands of catalysts.