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How to grout bathroom tub / tile joint?

Hello. I'm trying to fix worn out grout above my bathroom (white porcelain) tub. I dug out much of the old grout, and 1/2 of the botom tiles came out in the process. This row of tiles is cut to only about 3/4" high, and 4" wide. The spacing between tiles is 1/16", and 1/4"-1/2" between the tiles and the tub. Behind some tiles is a bit of the backing wood wall, and only empty space behind others. So, there's not much solid substrate behind. I've repositioned some tiles by gobbing in 'SimpleFix Pre-mixed Adhesive & Grout, for grout joints 1/16" to 3/8"' behind them on the tub, the bit of backing available, and between the new tile and it's neighbors, but not filling in all the way so i can do a surface grout after it sets (I thought it might shrink and not work if I filled in, adhered the tiles, and grouted all the way in one step.) For the tiles with only air behind them, I glued in thin 3/16" strips of wood onto the riser beams behind so that tomorrow I'll have something to glue/grout the tiles onto.
My questions are:
1. The "Adhesive & Grout" stuff is sandy, will I damage the porcelain (I think) tub, or the ceramic (I think) tiles?
2. Is this "adhesive & grout" a good grout for the 1/16" gap between tiles (I read it shouldn't be sanded), or the wider gap between the tiles and tub? (The ingredients are silica quartz, limestone, acrylic copolymer, water.) I read that normal grout wasn't flexible enough or doesn't bond the tub well enough to use at the tub/tile joint. If I should use caulk, what kind? My old silicone surface fix was getting moldy behind.
3. How about the joint where 2 walls come together? The existing had silicone behind caulk.
I have a few tiles that seem solidly in position now, but I have more to put in, and haven't grouted/caulked yet.
If anyone's actually read this, my thanks, and any responses are appreciated.

Re: How to grout bathroom tub / tile joint?

The bad news is that you have, based on your description, rotted wallboard behind your tile so you should understand that you're doing a temporary fix, the rot will continue and in a year you’ll have cracking grout and repairs again. Once mildew begins behind a wall, it doesn’t just go away. It’s always there until you remove the backer wall.

Beyond that, the “Adhesive & Grout” is a great product, it’s actually non-sanded so you can fill gaps 1/16th without a problem. The bad news is that it’s fairly water-soluble so the product does expect that you have a reasonable substrate in place behind the tile you’re grouting.

Essentially, you should remove the tile and wall behind it to the studs and replace. There is no other option that works. Otherwise you will repair this wall again and again.

Re: How to grout bathroom tub / tile joint?

Hi, thanks for the help.
I don't think the backing wall rotted away. I didn't explain completely, but it seems the original board installed behind the tiles did not reach the tub, but left a gap. The backing wooden board comes down to has straight edge that comes a bit behind some tiles, and not at all behind others which just had paper attaching them to the tiles above (I assume the tiles originaly came preearranged with paper backing). Doesn't sound like terrific craftsmanship, but the tiles did hold in place until I dug out the grout.
I don't understand what you say about the 'Adhesive and Grout'. I'm glad it's generally recommended. It really feels very gritty and sandy when I was applying it. Is it possible I have a different type, or is "sanded" something else. Also, why would they make a grout that was water soluble? Do you mean it's only slightly soluble so that if the backing wall is solid it won't be a problem, and if it's sealed it may not be soluble at all?
So, I'll assume the grit I feel isn't the same as "sanded", and it's OK to use on tub and ceramic tiles to glue my tiles in place, and for the gap between tiles. (Well, maybe I'll put some tape down to protect the tub as I ponderously continue.)
What about the larger gap between tiles and tub? Is the 'Adhesive and Grout' an appropriate product for that? I read that regular grout really was not but the movement of the tub would crack it; will this stuff work better, or do I need a different type grout or caulk? I read someone recommending silicone caulk behind grout. I'd have space to do this in the wider gap on the exterior of the tub, and where the 2 walls meet, but the gap looks so small at the top edge of the tub that I think if I put silicone in, I might have trouble then grouting or caulking over it, unless the overlaying product can bind the silicone well.
Incidentally, something seems to be eating into one of my fingers, although I was wearing gloves. Is this grout stuff caustic? I also tried a little acetone to remove existing silicone.

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