What is "boot-topping"

n. 1 (context nautical English) The process of cleaning the upper part of a ship's underwater hull and daub it with a protecting layer of antifouling substance. 2 (context nautical English) Any substance used for '''boot-topping''', especially a mixture of tallow, sulphur or lime and rosin, which was commonly used to paint the bottoms of wooden ships, as a deterrent against weeds and barnacles, and to reduce friction.