As a starter, try spraying undiluted H2O2 (3% Hydrogen Peroxide) directly on the desired area. Let it set for a few minutes. Spray it again. Soak it! Next... scrub with a slight abrasive sponge. GOOD LUCK !!!!!
If you don't have a marble tub, use "wet & forget shower". It's a no-scrub product that works wonders. I used it twice a week for 3 weeks to remove all the grime and gunk that had built up on my tub, shower, and doors. After that, once a week. Now they all look like new. Available at Lowe's, QVC, and walmart.com.
Try 20 mule team borax applied to a wet cloth and rub gently. You'll probably need a water softener system installed for your house.
I use a bunch of used fabric softener sheets soaked in vinegar.
Clean soap scum off your shower with Canola cooking spray
Who knew cooking spray could tackle soap scum buildup in your bathtub, shower, and sink? Since oil breaks down lime deposits, a quick spritz of cooking spray on your bathroom sink or shower tile will remove any gross grime in no time. Also leaves a coating that will repel water.
Just like nonstick cooking spray can remove tough grime buildup in your bathroom sink and tub, the same goes for cleaning your shower door, too. Simply spray a bit of oil directly on your dirty shower door to clear away the lime deposits and dirt real quick. Use a kitchen scrubby to clean off the crud. The fine coating left behind will repel water.
CLR and hot water with stiff scrub brush, I have nice stiff toilet bowl brush I use just for this it makes it much easier to get in crevices,edges,around fixtures or if glass is not smoothe. Put CLR on let it sit then put some more on brush & scrub repeat if necessary.If it's really bad may need to scrape gently with single edge razor blade. Do it regularly to keep from buliding up and making it harder to remove. Use white vinegar for regular cleaning also helps with build up occuring.
Thanks to all who responded so quickly! William, I tried using Pam Cooking Spray on a small section and to my delighted surprise it really works! I had already tried several other of the recommendations in the past two years but nothing else worked as well or as easily as Pam. On my way to Walmart to get more. I have two 8' tall, all glass shower stalls and since the previous owners never bothered to clean off the built up crud I will need a LOT of it! Thank you again!