This is the second pack in the two pack for Aquacote Polyurethane Top Coat. It is included when the paint is purchased but this is available if you need some extra. It is used at the rate of 1 1/2% (which is 45 drops per 100ml). The most remarkable property is that if you mix up too much top coat, you can return the unused paint to the can for next time as the crosslinker will deactivate itself after a few hours.