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Old 03-13-2018, 04:01 PM   #1
sossend
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Tiling over old mastic on plaster

Thanks in advance for the help. I'm replacing a backsplash in a 1950 kitchen. The house has plaster walls, not sure if they are on lath or blueboard. The studs are randomly spaced. (The story in town is that these ranch houses were built by Norwegian fishermen stuck here in the winter. So nothing is flat, square, plumb or level).

The old install was porcelain tile attached with old-style mastic right onto the skim coat. I know it's old mastic because, well, the house is old, and it melts under a heat gun then hardens up again in a few seconds. The plaster is in very good condition except for a few scrapes, gouges and holes.

The new tile is 1x1 glass on a mesh backing, 12x12 to a sheet

My plan is (or was) to attach Hardiebacker to the wall, then apply thinset and set the tile. The problem is, how to best level the wall and secure the backer board?

I do not want to go down to the studs. There is likely to be lead, asbestos and who know what else back there. The plaster is about 1 inch thick. And to top it off, I installed new counters and a new floor already (This was all after a fire. Free advice: do not buy a Thermador or Bosch dishwasher. Two house fires traced to these just on my street. I was lucky, I was home when it lit up. The other guy was not.)

My plan is to use thinset to fill up all the depressions and grooves in the mastic and underlying plaster. This should leave a pretty flat surface for the backer board. Since it is difficult to find the studs (if there even are any where I need them) I would sink brown wall anchors into predrilled holes (to help avoid cracking the plaster), then attach the backer board with Hardie screws and adhesive, like PL400. Then thinset and tile as usual.

Your thoughts?
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Old 03-13-2018, 04:35 PM   #2
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Welcome to the forum, Dave.

Is the plaster secure and stable, not crumbling or loose? If so, you may just be able to put a coat of primer and tile over that.

Before you run with that, can you post a few pictures of the walls from different angles so we can see what kind of shape it's in?
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Old 03-13-2018, 04:49 PM   #3
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Pix

Thanks. I'll take some pix and post them. The plaster is very solid (not cracked or crumbling) but it is not smooth or flat. I'd be concerned that if I applied thinset right over the old mastic it might not adhere, or, when I beat the tiles in and flatten the ridges, the tiles might tilt, wobble, etc.

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Old 03-13-2018, 05:13 PM   #4
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Pix

Here are some pictures with comments

(haven't figured out how to edit the pix, so the comments are all here, the pix are below)

Going from left to right

This shows the old mastic (brown) with patches of skim coat showing (white) and some of the gray coat where the tile pulled off the mastic and the skim coat. There may be two layers of mastic, see the later pictures

Next picture more clearly shows two layers (or at least two different colors) of mastic over plaster

The next picture shows one section where the tile was applied directly to dry wall. You can see the torn paper and one layer of adhesive. I guess this is yet a third kind. I plan to remove the drywall, then put up hardibacker right to the studs (if there are any).

The last picture shows a close up where there is clearly only one layer of mastic on top of the skim coat.
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Old 03-13-2018, 06:41 PM   #5
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You are right about the surface needing to be flat. With 1x1 tiles, the tiles will follow the humps in the surface.

I would tend to float it out with some patching material or even thinset if it's less than 1/4 thick. I would probably call Mapei to see if ECO Prim Grip would stick to old mastic. Here's a link to that product. http://www.mapei.com/US-EN/Tile-&-St.../ECO-Prim-Grip

Before doing anything, I would buy a 4 inch razor blade scraper to see if any of the mastic will scrape off. The big box stores has the scraper in the paint dept.
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Old 03-13-2018, 07:11 PM   #6
sossend
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Thanks. I spent some time scraping with a carbide scraper I use for boatbuilding. It flattened the bumps down some, but sloooow. I wonder if I could sand down faster, maybe with 40 grit discs? A lotta dust which might be toxic
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Old 03-13-2018, 08:40 PM   #7
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Yeah, I'd avoid creating a lot of dust. Scrape it as much as you can.
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