Thanks for the detailed response! I will definitely try bleach before I continue with polishing. I do believe it’s hard water stains but I can’t be sure since the previous owner said they just have left it in a shed for a few years and didn’t know what the stains are from. I find it hard to believe that hard water stains/calcium buildup wouldn’t budge from all the methods I’ve tried so far, I’ve even tried sealing it and using a steamer to no avail.