I think that the black slate 2 inch mosaic tiles used in the base of the shower was just the wrong material to use.
Check the caulking at the change of plane joints at the base of the shower.
I would re-caulk with a urethane or silicone black or grey caulk to prevent water getting into the pan.
Most sealers and topical will probably fail in this situation
You may want to try a product like stone glamour or sigma seal made by sparks,they may fail as well.
If it were may shower I would replace the base with ceramic faux slate-but larger tiles.

On Sat, Jul 2, 2016 at 8:29 AM, Barry Raduta <barry@diamondstoneandtilecare.com> wrote:
Hello partners and a Happy 4th of July to all.  I have a friend/ customer with a small black slate shower floor with 2" x 2" tiles.  Some of the tiles randomly have turned gray; and, in most cases, not the whole tile.   I was there about 6 weeks ago and stripped the floor to get it down to the state and be on the safe side  .  later I applied some color-enhancing sealer (penetrating), but within a week the gray tiles appeared again.  It is not efflorescence in my opinion, but the finish seems to have worn off.  I went there today to clean the floor and it will sit for 2 weeks so that it is totally dry.  I have never used a topical sealer, but I was wondering what you believe is a good approach to this problem.  Sorry no photos.

Thanks in advance

Thank You,
Barry V Raduta
Diamond Stone & Tile Care-South
Stone & Tile Pros

Visit list archives, subscribe, unsubscribe or change your subscription 
Start a new conversation (thread): 

Stu Rosen