Help!: What's this brown stuff on the wall?

[rsb123]

Removed old tile today and it took the skim coat (?) off the wall with the tiles, leaving the sandy brown layer, with concrete block/brick behind it.

wall.jpg

Questions:

1. What is the brown layer exactly?
2. If I intend to paint one of the walls, can I just skim coat it with something, sand, prime and paint? If so, with what?
3. For the walls I'm tiling, I assume skim coat = bad, so how about if I mechanically fix 12mm Hardiebacker and tile onto that? (12x18" travertine - heavy)
4. Assuming Hardiebacker, do I have to get every bit of old adhesive and skim coat off or can I leave the bits that are firmly attached and apply adhesive, then hardiebacker then screws?

Thanks

[whitebeam]

That looks like a gypsum browning base coat..

[peteablard]

It's either gypsum browning or bonding, both similar products which are basically base coats of plaster which are often put on brickwork prior to plastering. You'll probably find a lot more of the top coat will come off and your best to get as much of the loose stuff off as possible. Pretty straight forward for a plasterer to re skim

[Daz]

I'm deffo no expert on this but it could well be a sand / cement render that has been applied to block work and then skimmed. If the skim coat has blown, then get as much off as you can. This should easily detach with the use of a scraper.

If it is sand / cement then it will be absolutely fine to tile to. Just prep it with a couple of coats of acrylic primer according to the instructions on the bottle. I would use a cement adhesive and not tubbed ready mix too .

Daz

[user123]

It's the same stuff as used in the old lath and plaster walls, isn't it? I always just thought it was some kind of old plaster, it's pretty soft in a sort of creamy sandy sort of way, isn't it. And it always has smoother plaster on top. You can see I'm not plastering terminology expert, but remembering what it FEELS like I wouldn't tile on it directly.

[whitebeam]

If the back ground is brick or block work it would be a browning, if it was a plasterboard or a low suction (concrete or a surface coated with a pva) back ground then bonding would be applied.

MG, your talking a lime mortor background on laths and a lime finish which can be very powdery..

[rsb123]

It doesn't crumble off the wall easily, most of it is fairly solid and takes a good whack with a cold chisel to knock it off. But it's not solid as cement and I don't feel 100% comfortable attaching heavy tiles directly to it even using Mapei Keraflex Maxi. So I'm still thinking cement backer board screwed to solid wall (i.e. screws through browning into block).

For the painted wall, I will just skim it and paint though.

[user123]

If the back ground is brick or block work it would be a browning, if it was a plasterboard or a low suction (concrete or a surface coated with a pva) back ground then bonding would be applied.

MG, your talking a lime mortor background on laths and a lime finish which can be very powdery..

Great, thanks Mark, love to learn. You can tell I've done up old cottages, knocking down walls etc, can't you, never plastered in my life, just remember the 'lime mortar' (good eh? ) really well, and it does look just like it, remember it in my hair, under my fingernails, all over my overalls... up my nose..

[whitebeam]

And in your lungs, dust masks should worn at all times...naughty

[user123]

And in your lungs, dust masks should worn at all times...naughty

Nobody told me that xxxyears ago! Went a shade paler just then when you told me it was lime, didn't know that. Just stormed at it with crowbar and sledgehammer and ducked from the rebounding hammer and flying laths, with the old rusty nails sticking out, don't even think I had goggles then, nope didn't, oh dear...

[whitebeam]

Nobody told me that xxxyears ago! Went a shade paler just then when you told me it was lime, didn't know that. Just stormed at it with crowbar and sledgehammer and ducked from the rebounding hammer and flying laths, with the old rusty nails sticking out, don't even think I had goggles then, nope didn't, oh dear...

It's the lime that used, it's called hydraulic and if you get it in your eye when it's being applied, it burns badly

[user123]

I knew about the limewash I used on the garden walls that that was dangerous and covered up well, just didn't know that this plaster stuff was lime, too. But, a long long time ago, I lived, I'm well

[craigdon]

If it is a plaster undercoat, and it does look like browning from the picture, it shouldn't ever be tiled onto directly. As said, cement render undercoat can, once the suction is reduced by priming. If the weight of your tiles and adhesive don't exceed 20kg/m2 then reskimming and tiling would be fine.