There are a few products out there to use on the shell of your tub, such as Surface Cleaner (by SpaGuard), or Gel Gloss (don't know who makes it.). We sell both; the Gel Gloss is great for making the tub surface shine like new, and the Surface Cleaner is just that - perfect for a good cleaning between draining and refilling.
One word of advice, I highly recommend against using anything not made for a hot tub specifically. Many cleaners contain phosphates, which can cause your calcium to fall out of solution and continually build up scale. Phosphates are horrible to try and get out of a tub.
As far as the ring, that may have something to do with what type of sanitizing system you are using (BaquaSpa and Soft Soak are notorious for it). You can try using things like a Scum Bug or an enzyme to reduce/eliminate it. It also could be coming from lotions, hair products or make up.
Hope that helps!!!
