Flooring Tile - problem with new grout

I repaired several rows of loose tiles around a bathtub. I had pulled or pried off the old tile to find a half-inch plywood underlayment. The plywood was solid, with no water damaged areas. I removed the adhesive on the plywood, and wiped it down with mineral spirits. Then I used a ready mixed thin set to install the new tile. Grouted with unsanded grout, kept it damp for a few days, then sealed with a grout sealer. After about 2 months of use, most of the new grout has stained a brown color, and has several cracks. I'm going to drill out the old grout. But - before I regrout - any ideas as to what went wrong?

Why are you using non-sanded grout?

If your grout channels are wider than 1/8" you need to use a sanded grout. I would also reccomend that you modify your grout with acrylic admix. The admix will make your grout stronger, more dense, it will adhere to your tile better, and it will enhance the color of your grout.

I do have a few questions--- (1)is your tile installed directly over plywood? (2) is your tile ceramic or porcelain? Both of these questions are important to determine why your grout is failing.

Hi
That pre-mixed thin set(or mastic) takes forever to dry...your best bet would have been to replace the loose tile, and then let it dry a few days before grouting. Keeping the grout damp for a few days is also a step you can leave out, since moisture will soften the mastic under the tile...then as it drys out, it shrinks and causes cracks in the joints. I've heard about the brown staining before(but never seen it myself)...but my best guess is that the premixed stuff you used was not completly dry before you grouted.

I used non-sanded grout because the tile store told me to use it. The grout lines are no larger than 1/8. The tile is inexpensive ceramic 4x4, with some expensive ceramic listels included for accent. Yes, I did put it back on the plywood wall. This was a patch job - about 3 rows of tile around the tub. I couldn't really add any other backer because of the rest of the tile above the area to be patched.

thanks, adanac. that makes sense. I'll dig out the old grout with my dremel, then regrout, but will skip the misting part. Should I wait a while before I apply the sealer?

Hi
Your bottle of sealer will say on it how long to wait between grouting and sealing(from 24 to 72 hrs)...getting all the moisture out is important. You might want to check and see if the tile is loose...that bottom row at the tub is always a problem spot..especially with pre-mixed mastic, since it does soften if exposed to constant moisture.