I had a similar issue with my previously used toon that I over estimated how easy it would be to take care of. After 10 or so bottles of toilet bowl cleaner that took off about a quarter of the calcium build up, I took it in. The marina doing the cleaning tried many products and nearly gave up until they tried Zing bottom cleaner made for fiberglass boats. It formed up and broke up the calcium in no time and washed off clean. Then they used toon bright to even out the discoloration. Worked out great once they got the right chemical.I need to get the toons cleaned up below the water line. The previous owner left the boat in the water for the season for about 10 years. There is heavy build up (1/16" +) of mineral deposits. Im hesitant to use an acid like CLR or the works toilet bowel cleaner. Are those fine or is wet sanding with a 220 grit a better option? If I do sand what type of sander, da or line sander? Also what grit do I use?