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I built a shower about 6 years ago, and did the floor/wall joint with a sanded caulk that matched the wall grout color. I spent yesterday digging it out because it had pulled away in several places, and there were several places that it had almost seemed to calcify - the caulk was hard and brittle, more like grout than caulk. The pulling away seemed more pronounced in areas I had used foam backing rod.

I'm wondering if this is due to water sitting on it after showers. If so, might it help to grout with the colored caulk, let that cure, then put a thin layer of clear silicone over it? Most of the joints are 1/8"-5/16".

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I don't think you will have very good luck applying caulk over caulk (poor adhesion). I feel like your best bet would be to search out and purchase the highest quality caulk (rated for continual water contact) and just use that. Likely it would be something that contains silicone.

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It sounds like backing rod interfered with the proper function of the caulk. I realize that it's common to use that to save caulk in wide or deep gaps, but sanded caulk is already intended for use in wider gaps than unsanded caulks.

As long as your sanded caulk states that it is mold and mildew resistant and suitable for use in showers, I don't see a benefit to adding the silicone to the top. The silicone may also separate from the caulk below, leading to trapped moisture issues.

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