I'd like to replace my aging PVC tub surround with an inexpensive acrylic 3-panel one. Assuming I am down to bare studs after removing the existing surround (perhaps the backing wall will be damaged), what must I install on top of studs before I can glue the acrylic panels?
If it's a cement board, like Durock, Hardibacker, or Wonderboard, do I need to apply a waterproofing membrane (like RedGard) as well?
Greenboard is not a good choice for tile showers because grout will allow moisture to pass through. However it is a perfectly acceptable backer for acrylic panels. They are waterproof and no moisture will get past them unless you install them incorrectly. In fact, most such panels specify a drywall type backer since it is compatable with their adhesive. Cement board, Kerdi board, waterproof coatings, etc. are not only overkill, but may fail to bond with the adhesive the panel manufacturer supplies or suggests. Use the backer the manufacturer suggests! Frankly, if you are too paranoid to use the simple backer your panel supplier recommends, it is because you don't have faith in your installation skills. Stop reading so many "mine failed anyways" posts.
I use green board for this with redgard over in some cases (cheaper systems that don't overlap well). My take on it is if there is a water issue. I would rather the water be trapped in drywall and mold vs. drip down into stud bay and rot the wall or what is underneath it.
Taking out plastic sheets to repair a wall and adding more drywall is easy. The alternative isn't. Also my biggest concern with backer is the hollow sound it often projects when PVC surrounds are glued to it. Sometimes it sounds/feels like you are in a tin can.
This is all opinion though and you can go with backer or greenboard and regard or not. Local inspectors that I have dealt with don't care at all about whats behind tub kits. They are fine with putting it directly on studs. Really if installed right and caulked properly you shouldn't ever have a water issue. That is why I choose to go with what gives the best experience.
One disadvantage of direct to stud stalls is the lip at the outer edge over which the drywall will be applied - this causes a hump in the drywall surface. I usually fir out the studs with drywall shims so that the drywall lays flat onto the fiberglass or acrylic lip.
I would recommend using water resistant drywall for all the areas around the shower.
