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Unread 04-14-2019, 02:52 PM   #1
clifton clowers
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Tile over oil paint on plaster wall

Does anyone have any experience with tiling over old oil paint on plaster walls? This is in a kitchen, so not a wet area.

Laticrete suggests that it is a no-go saying the paint can turn to goo (they unhelpfully do not say how likely this is to happen) and that is must be removed. Though they also backtrack a little suggesting that sanding, or possibly heavier scarification is sufficient.

https://laticrete.com/~/media/suppor...131.ashx?la=en

Ardex also suggests sanding or scarification, and possibly the use of their primer. They don't saying anything about goo formation.

http://cdn.ardexaustralia.com/pdf/te...faces_2016.pdf

So, what would you do - sand? scar? prime? chemically remove? heat remove? lath and mud?
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Unread 04-14-2019, 03:09 PM   #2
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They don't say how likely your paint is to turn to goo because they don't know what kind of paint it is.

In my limited experience, paint adhesion to plaster is not something I would bet on. The paint all over my house peels off the plaster walls like cling-wrap. Plaster has to cure for a month before it's safe to paint and often that just doesn't happen. Even if ithat's not how it is in your kitchen, you still don't know how it will react to adhesive. I would definitely strip it off but it's your project.
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Unread 04-14-2019, 03:53 PM   #3
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What Paul said. I've seen thinset stick to paint that seemed to be bonded well to the wall and then for some reason the moisture from the thinset causes the paint to turn loose from the wall. I would test a scrap piece of tile on the wall to make sure it bonds well before tiling.
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Unread 04-14-2019, 03:56 PM   #4
clifton clowers
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Paint adhesion to plaster is actually pretty good in this case. The oldest layer is likely about 90 years old, and there is no peeling.

I put up a test tile onto a section of paint I sanded using modified mortar (X77). After about 5 days, I gave it a whack with a rubber mallet and it stayed up. A chisel popped it right off the wall. The paint stayed intact, but none of the mortar stayed on the wall. So absent the goo issue, it is the adhesion of the mortar to the paint that is the larger issue.
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Unread 04-14-2019, 04:11 PM   #5
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Hi Clifton. Yes, that's usually the problem. if it broke bond between the tile and thinset then I'd say you are good to go with that product.
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Unread 04-14-2019, 05:34 PM   #6
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How strong is the adhesion of mortar to plaster itself? I am feeling inclined to thoroughly sand and then to cut deep grooves every inch or so with a diamond blade (tile is 6x3 subway) to allow the mortar to form keys to make a mechanical bond. This feels easier than removing a lot of old paint, but I only if it works.
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Unread 04-14-2019, 07:55 PM   #7
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Try bonding a small piece to the wall and see how well it bonds after a few days, if you have time to wait.
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Unread 04-14-2019, 09:44 PM   #8
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I haven't had a kitchen since November - what's a few more days?

I tried some Smart Strip paint stripper. The initial application took off most of (the layers of) the paint (in one spot). I'll see if another round gets the rest.

That said, I have seen conflicting information about thinset mortar adhesion to plaster with some suggesting that mastic might be preferable. Any thoughts? Or just something I need to test.
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Unread 04-14-2019, 10:09 PM   #9
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I don't know of any manufacturers of mastic that list gypsum based plaster as an acceptable substrate.

Portland cement plaster, yes. But I'd bet that's not what you have.
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Unread 04-14-2019, 10:39 PM   #10
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I doubt it's gypsum - it's lime plaster.
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Unread 04-14-2019, 10:52 PM   #11
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Are you using small tiles? One thing I think, though admittedly it's just a hunch and my own logic, is that mastic is a lot better for painted surfaces for a backsplash/etc. I guess I think this as it's more similar to a caulk or glue. Also, you don't have to worry about the Portland cement and gypsum in the plaster reacting.

You're still obviously ultimately relying on the layer of paint's adhesion and all that, but I think with oil you're overall more safe, as latex will basically peel off stuff in sheets, whereas oil usually bonds well due to all the solvents fusing the paint with the surface (I mean, nail polish is essentially oil based paint, latex paint will come off in the sink with just hot water off your hands/etc) and dries really hard, but because of that, when it peels it tends to just sort of crumble and "alligator" as it's too brittle to allow expansion and contraction. If it's a super glossy oil paint, I'd actually go over it with an oil or shellac primer for adhesion sake, and avoid sanding it/messing with it due to the possibility of lead, but you could just test adhesion with the mastic and see if it's OK.

If it's small tiles and a kitchen backsplash I would just use mastic in this scenario, it's 100% OK as long as you wait a day or so for it to dry before grouting it. If it's big tiles, then you might have to do more drastic measures, but small tiles I'd keep it simple and just use mastic, especially since it won't be a wet area you'll be 100% OK.
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Unread 04-15-2019, 07:15 AM   #12
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Were this a few square feet of backsplash tile, I would not be worrying about it. But it is 250 square feet of floor to ceiling tile. So I want to be sure to get it right

The chemical stripper, left overnight, got down to the bare plaster without too much trouble, and with no collateral damage. That does not solve the tile on plaster issue, but at least it seems to solve the tile on paint on plaster issue.
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Unread 04-15-2019, 02:17 PM   #13
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It can also create another problem. A chemical stripper can be just as much of a bond breaker as the oil-based paint.
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Unread 04-16-2019, 12:20 PM   #14
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The stripper is water based and will clean up pretty easily.

I talked to Ardex tech support, and they said that tile over plaster can have issues if there is gypsum in the plaster as it reacts with the portland cement and can cause the tile to fall off the wall. Whether there is gypsum is hard to know, but they suggested P51 primer first (mixed 3 parts water to 1 part primer), and then tiling. (They also said Henry 564 primer is essentially the same stuff.)

Alternatively, they said that mastic would also work, but they thought that cement based mortar would be better in the long term.

Another question for the board - and perhaps another tech support call - of the 250sqft of tile, about 75% of it will be on Hardie with the remaining 25% on plaster. While I don't think I'd want to use it for the entire project, might an epoxy thinset be a reasonable option for the plaster part?
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Unread 04-16-2019, 10:16 PM   #15
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Kevin means that the stripper can cause a problem by driving the very thing you’re trying to strip from the wall deeper into the pores of the plaster where it’s still or causes a bigger problem. I’d leave the stripper alone.

Okay, you had a good, successful test with some mortar a couple days ago bonding a tile. I’m all for over engineering things, but epoxy setting material is “off the charts” crazy in terms of over doing it for a dry wall application.

Is the Hardibacker already purchased/installed? I ask because you’re the kind of person looking for a great bond. It’s easier to get a crazy-strong bond to a real cement board like Durock than it is to a super dense cement fiber board like Hardibacker.

Is your 250 sq/ft wall going to receive direct sunlight?

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