|Almost everybody who has ceramic tile
shower surrounds fights mildew. [ALERT: I
read what I'm about to say about soap scum in a web article, but cannot
confirm it's true.]
Mildew is a living organism. It requires both a growth medium (food/nutrient) and moisture to grow. As I understand it, the black grunge isn't the organism, but its excrement. So what's the mildew using for food on your shower walls? It isn't the ceramic tile and it isn't inert grout. The best candidate is soap scum. Soap scum builds up quickly on shower walls and provides great food for mildew. So, if you are going to ward off mildew, you have to get rid of soap scum. Unsealed grout and grout where the sealant is gone let soap scum soak into the surface of the grout, making it difficult to remove. So, after you clean/kill the mildew the food is still there waiting for the mildew, which is everywhere all the time, ready to land and start feeding. This will begin almost immediately after you remove it.
We don't towel off the walls after using the shower (Mildew requires moisture. If you keep the walls dry it goes a long way toward fending off mildew.) We only clean the shower wall infrequently. By the time we get around to it there's a substantial soap scum buildup. Soap scum is difficult to remove and ordinary household cleansers won't work -- soap won't remove soap.
To minimize the mildew problem:
If you do all this, you can probably avoid fooling with the grout for five years and maybe longer. You'll still have to clean the surface of the tiles if you want them to sparkle.
Removing Soap Scum
This article gives a way to remove the soap scum.
Here’s their recipe for a homemade soap scum remover:
Mix these together, put it in a spray bottle and apply to the area. Then scrub the grout lines with a narrow brush then rinse it off thoroughly. This OXO brush is made for the job. If you use a toothbrush get one with the stiffest bristles you can find. This cleans the grout -- you'll probably want to spray the stuff on the tile and scrub with a sponge to remove the scum from the face of the tile too. If you're going to do the tile face, do the grout last to avoid recontamination.
Killing and Whitening Mildew
You can try a bleach solution. This will kill the mildew, but in my experience doesn't do a good job of whitening the black stuff.
Here's a recipe for a bleach mildew killer:
Ventilate as best you can -- vent fan, ordinary fans blowing the fumes away from the area, open windows, etc. Breathing bleach fumes isn't the best thing you can do.
For this job I use a product called X-14. It's available a groceries. Depending on how long you've let the mildew grow, it may take two or three applications, but one usually does it for me.
Sealing the Grout
Sealing the grout prevents stuff soaking into it that nurtures mildew. If your tile was installed by a competent installer it was sealed originally, but the sealer typically lasts only about 3-5 years. Sealers vary widely in quality. This is a good sealer. If you apply it (tediously) only to the grout lines with a small brush you need a lot less sealer. You can apply it to the entire surface, let it soak in for five minutes and buff it off the tiles. This is quicker and easier but it takes a lot more and this stuff is a little expensive.