boot-topping

(nautical) The process of cleaning the upper part of a ship's underwater hull and daubing it with a protecting layer of antifouling substance.

(nautical) 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.

(nautical) The paint of the part of the hull that is designed to be underwater.