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we had a leak awhile back inside the wall, from a shower valve leaking. That has been corrected and everything is dried out.

Today while tearing out the sheet rock on the wall that backs up to the shower, I found black mold on the paper of the sheet rock that is attached to the shower faux marble wall. My contractor is telling me kilz will contain it.

Is spraying with Kilz going to keep anything from spreading?

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It has been proven in recent years that chlorine (bleach) does not kill mold, especially on porous surfaces. It hurts it a bit, but really just bleaches it out so you don't see it. That's why it comes back so quickly. The mold remediation industry no longer uses it and strongly advises against it. Here's one site (I just googled, do not know these people): http://www.countyservicesinc.com/bleach-myth.php

Sheet rock is one of mold's favorite foods and is quite porous - so if there is mold in a piece, it should be replaced. Kilz might contain it on other surfaces, but it will grow inside sheetrock and can spread to an area that isn't Kilz'ed, so I'd say "no" in this instance.

Also, in reference to James's statement ... chlorine does kill bacteria - but mold is a fungus.

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  • From their product description: KILZ MAX Clear contains a mildewcide that helps protect against growth of mold on the primer and is not intended to provide mildew protection on the substrate. => Mold or Mildew Covered Surfaces: KILZ MAX Clear should not be applied to a surface containing mold or mildew; it should not be used to cover up mold or mildew. Wash the area with a mildew remover, rinse with water and allow to completely dry before coating.
    – Fiasco Labs
    Jul 17, 2015 at 3:56
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Yes Kilz will probably contain it. Want to be sure? Take a spray bottle put some swimming pool chlorine in it and spray all the black molded area. This will kill the mold. Then, when it's dry, use Kilz on it.

Household bleach works well, too.

Safety Precaution

Do not mix bleach and ammonia; use one or the other NOT both!

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  • How would chlorine help? An explanation would better round out this answer.
    – alt
    Aug 26, 2014 at 12:18
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    I kinda feel like the idea that chlorine kills bacteria...not to mention many other things... (why else would you put it in a pool) is pretty common knowledge.
    – James
    Aug 26, 2014 at 14:04
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    Wear a mask and goggles. When it dries, get a shop vac with a brush and vacuum off the residue. Keep in mind that Chlorine forms lovely organic compounds that can be quite nasty - use the same precautions you would for lead paint removal.
    – Chris Cudmore
    Sep 27, 2014 at 12:47
  • +1 on the organic compounds issue, I can't drink chlorinated water for this reason as I somehow got sensitized to the organic byproducts. Makes travel miserable and no swimming.
    – Fiasco Labs
    Sep 27, 2014 at 15:43
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    @ken You mean "do not use both bleach and ammonia"; most bleach contains chlorine, so you could say bleach and chlorine come pre-mixed.
    – Daniel Griscom
    Aug 5, 2019 at 2:55
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Mold will not grow on dry surfaces under 55% humidity. If you contain the leaks and humidity remains under 55, the dormant spores will not spread. They will not die but they will not spread. If you use a primer sealer you will contain the mold spread.

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  • You may have noticed from the OP that this is a bathroom. It's highly likely that the humidity in a bathroom will be above 55% in many parts of the world, even for short periods during a shower or bath.
    – FreeMan
    Nov 2, 2021 at 12:23

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