# How to skim coat rock lath ceiling (with moldy area)



## Colbyt (Jan 27, 2014)

IMO, everyone who uses blown insulation should get to tear out a section of ceiling at some point in time so they can experience nuclear winter. 

Your plan sounds good except why Durobond? And I seriously doubt you get it smooth enough to suit you in just 2 coats. Ceilings are 5 times harder than walls to get smooth and flat looking.


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## Nestor_Kelebay (Jun 17, 2008)

Copyright said:


> I just want to verify my approach to the ceiling before I start.
> 1. remove the rest of the popcorn
> 2. sand the plaster surface to remove as much of the mold as possible (with exhaust so the airborne spores go out the window)
> 3. prime the surface with Zinsser Mold Killing Primer
> ...


Regarding #2: I would just use a cheap cellulose sponge to clean that area of the ceiling with bleach straight out of the jug. See if the mold disappears. If it does, the bleach has killed the mold.

NOW, there was a discussion in here a few month ago saying that bleach was ineffective in killing mold on porous surfaces because it didn't penetrate into that surface to kill the ROOT of the mold, and without doing that, the mold would grow back.

HOGWASH. If you put bleach on a porous surface like bare plaster, the plaster turns darker. It does that for the same reason that wet blue jeans are darker than dry blue jeans, and that's because the refractive index of water (which is what bleach mostly consists of) is closer to the refractive index of cotton than that of air. So, light bends less when it travels through wet cotton than dry cotton, and that results in the incident light going deeper into the cotton where it is absorbed. If your plaster turns darker when you dab it with a sponge wet with bleach, it's because the bleach is being absorbed into the plaster, and the refractive index of plaster is closer to that of water than that of air, so light bends less as it travels into wet plaster than dry plaster. So, if you see that your plaster darkens as you dab it with bleach, that is visible proof that the bleach is being absorbed into the plaster (where the "roots" of the mold are).

So, sanding the moldy area down would remove the mold, but so would killing the mold with bleach. And, after washing the moldy area with bleach, clean that area with clean rinse water to remove the bleach.

I would leave out Step #3. If you kill your mold with bleach, you don't need to prime your plaster with a bleach killing primer.

Regarding #4:
You don't need to skim coat the entire ceiling. I would only skim coat the area you sanded. To skim coat, simply spread a thin coat of joint compound over the area you sanded. Do this with a bright light shining at a sharp angle to the ceiling so as to exagerate the roughness of the joint compound so that you can SEE where you need to sand that joint compound down and where you need to add more joint compound in order to get a smooth surface on that area.

Regarding #8:
Please don't use ceiling paint. Ceiling paint is nothing more than wall paint that has been formulated to a lower price point because ceilings don't generally get dirty and so ceiling paints don't have to stand up to hard scrubbing to remove stubborn marks the way wall paints do. You'd be better off to use a top quality wall paint on your ceilings. BUT, BETTER STILL, use a paint made especially for bathrooms on both your bathroom walls and ceilings. I recommend Zinsser's PermaWhite Bathroom Paint which you can buy at Home Depot or any other home center. Apply multiple coats of PermaWhite Bathroom Paint and your bathroom walls will be mildew free for decades.

I own a small apartment block. I have 21 bathrooms in my building, and every one of them is painted with Zinsser's PermaWhite Bathroom paint tinted an off-white colour. It's important to the success of my business to ensure my bathrooms are free of mold and mildew, and I've found Zinsser's Bathroom Paint to be excellent at preventing the growth of mold or mildew on the painted surfaces in my bathrooms.

You don't need to use all those coats of primer. Once your plaster is painted with a bathroom paint like PermaWhite, the PermaWhite will kill any mold spores that land on the paint before they have a chance to grow. Mildew will not grow through the plaster to discolour your ceiling from above because unless you have a roof leak, there won't be a source of moisture above your plaster, and mold and mildew need moisture to survive and grow.

Hope this helps.


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## ToolSeeker (Sep 19, 2012)

Forget the Dura Bond. PITA to mix, hard to sand, hard to work with, hard to clean up tools, and you don't need the extra strength so no advantage for all the extra work.


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