Use an acrylic based caulk from someone like DAP. I'd say it's pretty water resistant. Silicone cannot be reliably painted and if you have water regularly getting up at the top of your surround, like another poster said you have bigger issues. Source - have done showers of many types for years Answer from Jasper-Jack on reddit.com
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GE Sealants
gesealants.com › home › how to seal a shower to prevent mold & leaks
How to seal a shower in eight steps to prevent mold and leaks
July 22, 2024 - Seal in the corners and seams where walls touch each other or the floor. Seal around fixtures such as the shower head, faucet handles, drain, and spout.
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National Plastics
nationalplastics.co.uk › blog › post › how-to-seal-shower-wall-panels.html
How to Seal Shower Wall Panels | National Plastics - Advice and Guidance - National Plastics
November 26, 2024 - Sealant finishing tool. Firstly, put on your protective gloves. Silicone is an irritant and may cause a reaction on your skin. Remove any existing silicone using a sharp knife and then wipe away any residual silicone with a clean, damp cloth, trying not to scratch the surface of your shower tray or wall panels.
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Care And Repair
careandrepair.com › home › tips and ideas › how to seal and waterproof your shower: the beginner’s guide to a leak-free shower
How to Seal and Waterproof Your Shower: A Beginner's Guide
May 1, 2024 - Make sure all seams overlap by at least 2 inches. Apply silicone-based sealant around the edges of the shower, including where the walls meet the floor, around the drain, and at any joints.
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BigBathroomShop
bigbathroomshop.co.uk › blog › how-to guides › how to seal a shower tray the right way
How to Seal a Shower Tray the Right Way | Big Bathroom Inspiration
January 16, 2024 - High quality silicone sealant– make sure it’s designed for bathroom use and that it contains an anti-fungal additive. Use the right type of sealant – if you’ve used natural stone for the floor and wall coverings ...
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YouTube
youtube.com › home repair tutor
Waterproofing Tub Shower Walls | Easiest Method for Beginners - YouTube
This video shares how to waterproof tub shower walls and is arguably the easiest method for beginners. For more home improvement tips visit https://www.homer...
Published   August 14, 2020
Views   1M
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YouTube
youtube.com › watch
How to Caulk a Shower the Right Way to Prevent Mold & Leaks - YouTube
Old, damaged shower sealant can let water and moisture into your walls and create serious problems like mold, mildew and rot. To prevent this, it’s important...
Published   October 4, 2022
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Wikihow
wikihow.com › home and garden › home maintenance › bathrooms › showers › how to waterproof a shower: 13 steps (with pictures) - wikihow
How to Waterproof a Shower: 13 Steps (with Pictures) - wikiHow
February 24, 2025 - After building or stripping the walls of the shower, you can utilize a combination of liquid waterproofing adhesive and tightly woven reinforcing membrane to make the area surrounding your shower immune to water damage.
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GE Sealants
gesealants.com › home › how to seal a shower door correctly every time
How to seal a shower door Correctly Every Time - GE Sealants
July 22, 2024 - These will apply to all shower door designs you might encounter as a contractor when you seal a shower : Use 100% silicone sealant because it’s waterproof and durable. Clean the area before applying sealant.
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DBS Bathrooms
dbsbathrooms.co.uk › news › how-to-seal-a-shower
How To Seal A Shower | DBS Bathrooms
September 5, 2025 - Use protective gloves because silicone is an irritant and be careful not to damage or scratch the surface of your shower panels/tiles or shower tray. Use white spirit to remove any small strands of silicone. Clean and dry the surfaces you are working on. If there’s any moisture beneath the silicone, it will become a place for bacteria and mould to grow. Use masking tape or duct tape to isolate the groove you are about to apply the silicone to, leaving just the area where the sealant needs to go. When you fill the groove between the wall and shower tray with silicone you don’t want there to be any unfilled space remaining, which is why the next step is to cut the silicone cartridge at a 45-degree angle so it’s slightly larger than the gap you’re about to fill.
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Wall Panels World
wallpanelsworld.co.uk › home › blog › how to install shower wall panels | diy | watertight seal
How To Install Shower Wall Panels | DIY | Watertight Seal - Wall Panels World Ltd
Clean the wall corners thoroughly and wipe with alcohol to remove any grease. Apply a zigzag bead of panel adhesive to the back of each trim piece. Position each trim, ensuring it sits perfectly level (use a spirit level to check).
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YouTube
youtube.com › home repair tutor
How to Waterproof a Shower (3 Awesome Methods!!) - YouTube
What's the best way to waterproof a shower? You'll find out. These 3 methods make remodeling a shower a lot easier. If you're doing a bathroom remodel and yo...
Published   February 9, 2016
Views   2M
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EVO-STIK
diy.evo-stik.co.uk › help-advice › how-seal-shower
How to seal a shower | Best way to seal a shower | EVO-STIK
Starting in a corner, place the nozzle at a 45-degree angle from the surface and slowly apply the silicone, maintaining pressure so the flow is constant as you work from one end of the shower to the other. Good to know: If you can see small waves on the top of the sealant, that means it is overflowing and you have filled the gap between the surfaces.
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Sika
zaf.sika.com › residential and home improvement › step by step diy projects › how to waterproof a shower
How to Waterproof a Shower
The most trusted solution to prevent ... agent, Cemflex®. Cemflex® mixed with Portland cement and water forms an effective waterproofing slurry that is used in conjunction with the Cemflex® membrane....
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Loctite
loctiteproducts.com › homepage › know how › fix stuff › how to seal shower grout quickly and neatly
How to seal shower grout quickly and neatly
Want to learn how to seal shower grout and keep your shower looking pristine? We show you the best way to seal shower grout, step by step.
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Harkraft
harkraft.com › home › how to properly seal your new tub or shower
How to Seal a Shower or Tub: Expert Tips
February 23, 2024 - You’ll need to use caulk designed for the shower anywhere water will settle, such as where the wall meets the lip of your new soaking tub, for example, or along the wall seams. As we mentioned, different materials will require a different type of sealant.
Top answer
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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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Homebuilding
homebuilding.co.uk › how to
How to silicone a shower if your old seal needs replacing due to mould damage
December 20, 2024 - Depending on the size of the gap between shower edges and walls and floors you should only need to add one run of silicone. But if you have deep edges, fill these with a small bead of silicone and run a wet finger along to make sure the silicone ...
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Fine Homebuilding
finehomebuilding.com › three ways to waterproof tile showers
Three Ways to Waterproof Tile Showers - Fine Homebuilding
August 5, 2021 - Plumbing penetrations are prone to leaks and water infiltration, but sheet-membrane systems provide great products for sealing these weak points too, including collar seals that go around pipes, and gaskets that seal around the mixing valve. Also, you can use the standard seam strips in nonstandard ways: around the edges of niches and curbs, and even around the entire floor—taking advantage of their strength and crack-isolation properties. Foamboard systems can be used for walls, the pan, or both to create a shower substrate.