How to Clean and Seal Granite Countertops. Monday, . How to Clean and Seal Granite Countertops. Natural stone is an investment..Granite countertops, like other natural stone countertop materials such as, marble, sandstone and slate are resistant to staining and easier to clean only when properly sealed..Why should you seal granite? Sealing granite is such a misunderstood practice and misused marketing weapon. It’s the result of ignorance in the stone industry and malicious intent by sales people of competing countertop materials..Step Test for Proper Seal. Once the entire granite surface has been sealed and wiped down, it will dry to the touch within three to five hours. Allow it to fully cure for hours before placing appliances and accessories back on the counters. You can check to make sure the granite sealer worked by sprinkling a bit of water on the counter..