Use a clean kitchen towel to dry the slate countertop completely. Apply a pH balanced cleanser designed for natural stone. Clean the slate with warm water and a microfiber cloth or soft scrub pad. Use enough cleanser to create a rich lather on the cloth or pad. Wipe the surface dry with another clean towel.
Apply a stone enhancer and sealer product to the countertop with a mop or lambswool applicator. Manufacturers' instructions for such products generally advise users to apply the product "liberally," though excess sealer should be wiped up with a clean rag to avoid spills. Allow the sealer to settle on the slate for about five minutes, then apply a second coat. Wipe up any excess and allow the second coat to dry for four to six hours.
Clean all applicators and your hands with warm, soapy water immediately following the second application. Don't clean the countertop for 24 hours. Apply mineral oil if desired to the slate surface for shine and maintenance purposes.