The vessel surface or finish of most manufactures is acrylic which comes in sheets like plexiglass. This sheet is heated to make the acrylic pliable and is then vacuum formed to a prototype vessel, allowed to cool and removed for fiber glassing the back side along with blocking and supports. Then it's off to the pipe works department.
The most important part of cleaning Spas and Hot Tubs is to thoroughly remove all cleaning solutions from the surface by rinsing with water and extracting with a wet/dry vacuum and repeating if necessary. The cleaners used are not meant to be part of your water chemistry when the vessel is filled for use and will significantly alter test readings and balancing procedures.
Before using Commercial Cleaners try:
Vinegar and Water
Never use window cleaner, acetone or ammonia based products on acrylic.
Do not use automotive type waxes on spa or hot tub finishes below the water level, water chemistry sanitizers used for maintenance will strip them off.