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We have an old house, (170 years).

The last owners grouted the bath to the wall and then caulked over it.

It's now black and mouldy. If I remove the caulk and bleach the grout is it ok to caulk over or should I completely remove the grout before caulking it again?

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    Is the grout loose at all? The reason you don't grout between two angled surfaces is that when the surfaces move the grout cracks and loosens. – spuck Mar 2 at 17:27
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Mouldy material should be completely removed - sometimes there is more mold behind it.

The tub should be filled with water before caulking in order to avoid pull forces. Push forces are less problematic to caulk. A slope of 25 degree or more prevents standing water on the surface of the caulk, which is a frequent reason for mouldy caulk.

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  • Thanks. Ignoring the mould, is it ok to caulk over grout? i.e, if there were no mould and the grout is a bit cracked is it ok to caulk over? – SeanJ Mar 2 at 14:10
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    Yes, a good quality silicone caulk (I recommend silicone for this job, BTW) will stick well to clean dry grout. – Jimmy Fix-it Mar 2 at 15:31
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If I remove the caulk and bleach the grout is it ok to caulk over

Yes. Make sure you clean everything well, and scratch the surface of the grout if it is mouldy too.

Once you've done that, crouch in the tub and shift your weight up and down to see if the crack widens. This is to check if whatever 170 years old structure the tub is resting on is still solid. If you see it widen more than a few millimeters, perhaps you need to check under the tub for rot. If it doesn't move at, or just a hairline crack, then it's good.

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I've drawn the caulk in red. If you don't make it thick enough, it'll make a dip where water accumulates (in blue) and that makes mold grow much quicker. It will last longer if you put more caulk to fill the dip so water doesn't accumulate. A tool like this helps.

I use water-based silicone, it is a lot cleaner to use than previous generation non-water based ones which tend to stick everywhere and make a mess.

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