Actually, although it may be too late the best way to avoid mold and mildew is to simply waterproof the shower from underneath.

It’s cheap and simple but unfotunately most tilers don’t know about it or do it. All yu do is paint the cementboard with redguard, (buy at home Depot) Laticrete 9235 or use a waterproof membrane.

Although this sealer may help there is no sealer that will make grout, which is porous completely waterproof. (I’ll check this one out though.)

There are many pitfalls with tiling. You can educate yourself about many of them by going to our 2 websites at:

http://www.groutrestoration.net

http://www.tileboston.net

I hope these help you with future tiling projects.

Lane