Color sealing will look great in a shower – for about a day. As Mike said - - - way to much water - - -.
Sort of unrelated; I once read somewhere about the approximate “Rainfall” per year in a shower. The study likened the amount of water that runs through the average shower over the course of the year and compared it to “typical rain fall”. Has anyone here seen anything in print like that before?
Back to the question, I don’t recommend color sealing a shower or any other very wet setting, I think it will fail in a fairly short time. (But I have heard of some doing it.) Your best bet is to cut out either just the offending grout or all of the grout so the whole shower will match. You could go the way that many “Redo” contractors do and just barely scrape the surface of the grout and then regrout it. Again, it’s not going to last long if you do that. You really need to make it a point to scrape out at least 50% of the depth (the thickness) of the tile and then regrout. This will give you the best results. One more point; okay, two more points.
1 – In your transitional grout line; i.e. where one all meets another or where the walls meet the floor or where any one plain meets another (bench) there will always be movement in the and the grout WILL crack. The way to overcome that is by caulking those areas. Run a bead of color matched siliconized caulk in those transition grout joints. If you can find siliconized caulk in the right color get a latex caulk the correct color and then go over it with a clear silicone.
2 – I would really encourage you to seal the shower when you’re done with the regrout with a very good quality penetrating sealer. You will need to wait 72 hours before you seal it so the grout can cure properly.
I hope this helps a little.
David Gelinas
Marbleguy