Unless you cut all the old caulk or grout out of the corners first you are not going to stop the discoloration from showing through no matter what you use. But once you have done that you can clean the shower with any grout and tile cleaner available at your grocery store or local hardware store. You can also make you up a solution of bleach and water (about 5 to 1 water ) or if you have colored grout joints you can use the same ratio with white vinegar. But the key is getting all that old stuff out first. An excellant product for everyday maintainance is Shower Fresh. All you do is take your shower and when you step out spray the walls down with is and walk away. It will help prevent soap scum build up and hard water deposits. One other thing you can do if the shower is in otherwise good condition is to buy a $ 10.00 bottle of grout stain or grout renew and paint the joints. I t may sound strange but I have used it on a many of shower stall and tub surround and saved the customer literally thousands of dollars.
Robby
June 2006