I put travertine tile into my shower over a year ago, grouted it, sealed it all really nicely, etc. To this day, the shower stays quite clean except for 1 or 2 tiles that must've been a little too porous. I didn't smear the sandless grout over the face of the tiles like I should have to fill in tiny holes but I did seal it twice before using it. Now I'm noticing some of the small pores in the niche bottom (it constantly has soap on it and is always wet, so no surprise there) and one random tile on the back wall are getting mold/mildew into them and I can't clean it out with a toothbrush or nylon brush. I want to get it cleaned before it gets worse so I can put grout into those tiles and re-seal again. The problem is I have no idea how to clean it. Regular nylon brushes don't seem to do anything at all. Is there a special fine-tooth brush or chemical I can use that will work?
2 household chemicals come to mind both work well for killing mold spores. House hold bleach and hydrogen peroxide. Both have the possibility of changing the color, I prefer hydrogen peroxide because it doesn't smell as bad. I learned about using Hydrogen peroxide when working at a hospital for a few years they would purchase stronger than the normal store bought type and add it to water then wipe the infected areas down. Since finding this out I have used a 3% solution to remove mold from carpet and spray directly on wooden surfaces with better results than bleach and less damage (discoloration) without the smell of bleach. The store bought Hydrogen Peroxide will work with a good saturation of porous surfaces.