"Finally, if you do select one of the "plastic" units, you can keep it looking new with a little known trick. Once a month, take just fifteen minutes and apply a high quality spray automotive wax to the wall surfaces only. Never apply wax to the floor of the unit. Buff this wax out and it will be as shiny as new!"
http://www.askthebuilder.com/065_Acr..._Showers.shtml