I have an old hot tub that I'm refurbing I guess. I was wondering what is the best way to remove the scale deposits ? I bought a cleaner from a pool supply store but it barely touched it. I lived with a surfer pool guy and we acquired an old spa and we wet sanded the interior to clean it up a bit and it seem to do a good job and the tub didn't have a glossy finish anyway. is this adviseable? anyone have a good suggestion?