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Unread 01-22-2023, 02:46 PM   #1
mdb06
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Marble mosaic backsplash - removing paint on drywall?

Appreciate the info on this site. For my next project, it will be first time with marble backsplash. I previously did the kitchen backsplash with 2x8" ceramic tile over painted drywall, using Custom's Omnigrip lightweight tile adhesive. I've now learned, contradictory to my research, that tiling over painted drywall was a no-no.

So now I'm doing 8 sq. ft. of bathroom vanity backsplash, white marble mosaic tile, 12"x12" sheets, each tile 1"x1". My questions: how should I approach tiling over the existing painted drywall? What thinset should I use? If I am able to keep the existing drywall, what's the best way to strip the paint off and is going down to the paper drywall layer enough? Do I need a sealer over the drywall before tiling?

From research: use white modified thinset. Seal marble before grouting. Add extra drywall screws to make sure the drywall is anchored well to the studs, if in doubt.

The Custom rep says I can use my white Omnigrip lightweight tile adhesive for this project, but that I need to strip the drywall of paint. I'm just not sure how to go about that, and have concerns that using the wrong paint stripper will weaken the drywall paper/board. Thanks for any advice.
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Unread 01-22-2023, 03:57 PM   #2
Tool Guy - Kg
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Welcome to the forum, Michael!

Mastic over painted drywall in a backsplash application rarely yields a problem. The biggest problem I’ve had is that the mastic temporarily softens the paint. If you go to remove a tile a half hour or more later, you might pull off the tile, mastic, and layer of paint in one shot. Not a big deal, clean off the tile and use more mastic to set on the now perfectly clean square. This might worry you at first, but once everything dries, the install is solid…it’s no longer an issue.

But the Custom rep might be steering you wrong saying it’s okay to use on your natural stone. Some natural stone is stained by some mastics. If this happens, the only practical way to fix the stained tile is to replace it.

What you want is either a mastic specifically rated for use with natural stone, or a very sag-resistant thinset mortar. To prepare your painted drywall surface, just rough it up with a heavy grit sandpaper in the “footprint” of where the tile will go and clean off the dusty residue. You don’t need to go through expensive prep. Though, if you’re allergic to your money and insist on spending more, you could cover the paint with Custom’s Multi-Purpose bonding primer or Mapei’s Eco Prim Grip or another such coating. In the case of a backsplash that will see only mild splashes of water, I think it’s a waste of resources to use it.

As far as the weight, there isn’t an issue. Add screws if it makes you feel better. But a single drywall screw is more than enough to hold the approximate 30 pounds of this new backsplash.

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Unread 01-23-2023, 08:51 AM   #3
mdb06
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Thank you for the welcome, and thank you for the info, Tool Guy.

Your explanation and reasoning make a lot of sense. Appreciate the detail! I like to keep projects simple, but won't go as far as cutting corners. I'll be sure to rough up the textured (orange peel) painted drywall, without damaging the drywall outermost paper layer.

As for the Custom product TDS:
"Suitable Tile Types: Vitreous, semi-vitreous or non-vitreous tile: ceramic and impervious porcelain, mosaic, quarry tile, slate and stone."

That's odd to me that it could work for impervious porcelain as well as a stone such as marble, a "non-vitreous tile".

But like you said, I've read enough about the argument for thinset for marble install that I'm leaning toward a high-polymer modified thinset for this job to avoid issues. And for that reason, I'm willing to fork over a little extra $$.

Thank you again for the advice.
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Unread 01-23-2023, 11:53 AM   #4
jadnashua
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Mastic is more of a glue so it can bond to numerous surfaces. Cement based mortars bond mostly by literally growing crystalline, interlocking spikes into any minor surface irregularities in the materials . Cement is more like Velcro that eventually hardens to hold things in place. The initial bond is more like suction. That's why movement with cement based materials can cause them to fail...it breaks those crystalline spikes. A modifed mortar will have some adhesive properties but also can cushion the cement's crystals.
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Unread 01-24-2023, 09:42 PM   #5
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For your non-sag mortar, be sure to mix per the directions. Non-sag mortar is routinely over-watered by DIY’ers and pros using it for the first time after being accustomed to how they’ve been mixing thinset mortar previously. Non-sag mortars go from wonderful to crappy if you add too much water.

Do you have a particular mortar in mind, yet?

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Unread 01-25-2023, 12:06 AM   #6
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Also note that the texture and workability of mortar can change RADICALLY if it is not mixed properly. Some can change significantly as they are mixed, and won't appear to be okay until they're mixed, so it's easy to overwater if you look at it only in the middle of mixing.
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Unread 01-31-2023, 08:46 PM   #7
mdb06
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Appreciate the additional tips. Not pro level, but getting used to mixing mortars after several projects. Seems like major rule is don't add more water after slake process. Also mix thin enough to peanut butter consistency, but must still hold notches when troweled...to name a few.

Tool Guy: I've decided on Custom's "Natural Stone & Large Formate Tile Mortar" (formerly "Marble, Granite, Travertine Mortar") color: white.

It's modified and seems it should be plenty adequate for my 12x12" mosaic marble that is still quite thick and heavy.

Debated between the custom's pro-lite mortar, and custom's ready-mix thinset. Figured my money will go further and I'm able to use the other half of bag in a few months on another project.
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Unread 01-31-2023, 09:10 PM   #8
jadnashua
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If you think about it this way, per square, it doesn't matter the size of the tile, once it's up, it's the same weight when done. The 'sticktion' of the tile to the surface per square measure is the same.
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Unread 01-31-2023, 11:00 PM   #9
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Quote:
Originally Posted by Michael
Debated between the custom's pro-lite mortar, and custom's ready-mix thinset. Figured my money will go further and I'm able to use the other half of bag in a few months on another project.
You wanna be careful with that, Michael. Thinset mortar manufacturers generally allow one year from date of manufacture for use of their product, presuming an un-opened bag that has been properly stored in a cool, dry place. Once that bag is opened, all bets are off. A "couple months," even with somewhat careful storage, could make a difference between a fella being penny wise, and pound foolish.

My opinion; worth price charged.
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Unread 02-01-2023, 10:22 AM   #10
mdb06
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Makes sense Jadnashua.

CX: Thank you, and this is a topic I've read about and realized not to gamble with. Has lead me to never buy half bags of mortar at the habitat restore and get ride of all my partial bags of mortar from a year ago! But I appreciate the awareness.
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