I apologize for the delayed response. It really depends on the shell. It could be calcium buildup or just the shell itself. On our shell we used 800 grit sandpaper wet by hand and with a soft sponge. After it is dry you can use white rouge compound bar and polish/buff pad from Granquatz. White marble polish cream might work as well. Careful of excessive heat. This may cause shel to burn or discolor...Hope that help!