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The "seal" between my shower base (fiberglass) and the surrounding tile started to crack (it is around 5 years old). It is not regular caulking, it's more like mortar/cement. It cracked a little bit and some areas fell.

Mortar

Should I re-seal it? If so, what would be the recommendation? Remove the entire mortar seal and replace it with caulking, or just apply caulking on the areas where the mortar cracked?

UPDATE: Based on the answers and comments, I think there is a lip that won't let water drip into the wall, here's a picture of the corner of the base. There's a "hole" like this in two of the corners of the base:

Corner

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I would not use acetone because a spill (which is very likely) could (would?) damage the fiberglass shower pan. If you feel it necessary to use a solvent, use rubbing alcohol, and be sparing with that.

I doubt a solvent is necessary at all. Note that the shower pans have a lip behind the tile so water is almost certainly NOT leaking into the wall. So there is no urgency. It would be unwise to recklessly do the wrong thing in an uninformed attempt to caulk this joint. DON'T think, "I have to do something now, the problem will only get worse and worse the longer it goes on."

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There are two schools of thought: One is to attempt to maintain a waterproof 'seal' by ensuring all grout is treated with a waterproofing agent (but that agent is likely to break down and need to be reapplied after 5 years). The other is to assume that any waterproofing will eventually fail, and that one should try to channel any moisture back into the tub.

In either case, the base of the tub can be grouted as shown in you picture, but the grout won't stick to the fiberglass and when you step in and and seal it provides will be broken. Thus if you want a true seal you should use a flexible silicone caulking.

However, moisture can be absorbed by unsealed grout and into the cement backer board, and when you take a hot shower, the temperature differential can cause condensate to form on the back of your tiles. The lip in your picture is called a "weep hole" and is intended to allow that condensate to flow out from behind the tiles and thus is not intended to be covered with caulking or mortar.

The effectiveness of weep hole is contested because if your tiles aren't troweled vertically, the condensate won't be able to flow down to it. Moreover, your specific setup is questionable as you likely are better off having small weep holes every 12-18 inches than just 2 large ones on either side of the tub.

Thus, my suggestion would be:

Carefully clean out loose grout using a grout saw (try not to scratch the tub).

  • If you are sure that your tile was installed using vertical troweling, then re-grout the tub everywhere but where your weep holes are (and possible leave an extra spot in the center of the wall), then seal all grout with a grout sealer.

  • If you aren't sure you have vertical troweling, then don't bother rerouting the base of your wall, but re-seal the grout in the rest of your wall with a grout sealer, then use a silicone caulk around the entire base of your shower. Remove and replace this caulking about once a year.

See this answer for more detail about tile tub interfaces: https://diy.stackexchange.com/a/67538/18575

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You can use silicone instead (white should not be ugly) to re-do all the chaulking around the shower plate.

TIP: to do a nice finish 'soap' your finger with some liquid soap and pass it over the silicone.

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Your fiberglass shower base will slightly flex when you step in. Cement-based grout is not flexible at all, so it will crack. Besides, it doesn't stick to fiberglass, resin, etc, only to tile.

Solution is to use a flexible material like silicone caulk. However the joint needs to be large enough to fill properly and ensure proper elasticity: the wider the joint the more elastic it will be.

So you will need to remove the existing grout, then give it a good cleaning. You can use a rag dipped in acetone placed at the end of a spatula or trowel to clean the inside of the joint. Protect the fiberglass with tape first, as it may react adversely to the acetone.

And please get one of these:

enter image description here

it will make your job a lot easier...

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    I would not use acetone because a spill (which is very likely) could (would?) damage the fiberglass shower pan. If you feel it necessary to use a solvent, use rubbing alcohol, and be sparing with that. I doubt a solvent is necessary at all. Note that the shower pans have a lip behind the tile so water is almost certainly NOT leaking into the wall. So there is no urgency. It would be unwise to recklessly do the wrong thing in an uninformed attempt to caulk this joint. DON'T think, "I have to do something now, the problem will only get worse and worse the longer it goes on."
    – Jim Stewart
    Mar 27, 2018 at 11:51
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    Yeah if it has a lip, it'll be a lot easier. Last one I had trouble with didn't have a lip, so water just poured out! If acetone is too hardcore, I'd use a piece of sandpaper bent in two at the end of a spatula, this makes it easy to remove the dirt inside the joint.
    – bobflux
    Mar 27, 2018 at 13:01
  • @JimStewart There might be a "lip", I updated the question with one more picture. Actually, your comment is the best answer to my question. Would you mind writing an answer with it, so I can accept it?
    – Viccari
    Mar 27, 2018 at 20:50

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