Skim plaster over mixed surfaces - undercoat/bonding?

Soldato
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TLDR: What do I fill these larger holes with before skimming, and do I use bonding, undercoat plaster, hardwall, or just skim with multifinish?

Currently doing our bathroom. All the walls except one are plasterboard, so they're about to get boarded, but the last wall is render/plaster over brick:

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The bit that's currently got the old pinky paint on it is going to be the 'show' bit, everything below the bottom edge of the window will be tiled so it just needs to be flat. I've hacked out all the loose, so everything that's still there is solid. There's a pretty big hold below the window - bit unsure what to fill this with. Just a bit of sand and cement? I'm probably going to sand and cement the top jaggedy part in where I've replaced the ceiling too.

I've skimmed a wall that's similar to the top half of this one, and I just got all the loose off, PVAd and skimmed it, came out fine - but this is quite a bit rougher and is really mixed surfaces.

Do I use bonding for this, or undercoat, or hardwall? I can't really work out what's appropriate, they seem similar. Can anyone fill me in (no pun intended)

The bathroom is extremely tiny so I don't want to batten and plasterboard, or dot/dab and plasterboard, I don't think. It's about 1200mm from the door to the wall! Every mm counts.

Thanks
 
Soldato
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I had similar sort of holes in a hallway after removing a dado rail and I did the following:

- Removed the loose bits and debris
- PVA (I just did 1 part pva to 3 parts water I think)
- Thistle undercoat plaster to about 3-4mm shy of the level of the wall (I keyed the undercoat to give something for the next layer to cling to)
- PVA again
- Thistle multifinish to level with the rest of the wall

One lesson I did learn was the undercoat is an absolute nightmare to sand. I thought I'd be lazy and use it to fill some less noticeable holes level with the surface instead of finishing with multi finish. If it's even 1mm proud, it's an absolute pain to sand level.
 
Associate
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28 Jan 2006
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pva then bonding coat for the holes. green grit the whole wall then skim it. but as your tiling some of it, how heavy will the tiles be, i wouldn't be happy putting porcilain tiles over that so would knock the lot back to brick and dot and dab it, only skimming down as far as the tiles are coming up to. also on the stud walls use tilebacker boards over the bath/shower area, or if you use plasterboard tank it in any wet areas.( i'm a tiler who also does quite a bit of plastering)
 
Soldato
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If you are tiling I would probably board that wall out. There are cementius products as well as foam extruded.

You can directly affix to the wall if it's mostly flat or batten out to level and affix to that.
 
Permabanned
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SBR or PVA and patch the deep holes with bonding coat. Allow to dry out for at least three days.

If you are tiling then you don't need to be too precise so there is no need to skim coat. Score through the paint until you hit plaster ( not really necessary with emulsion but I like to do it because you never know what's under the emulsion! ). Remove any high spots or lumps and bumps and apply an overall coat of SBR or waterproof PVA. Apply your skim coat or tiles. BAM! Done.
 
Associate
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The plasterboards walls I would at a minimum putting green boards up

no benefit at all in using moisture resisteant plasterboard on bathroom walls, wet areas would still need tanking and dry areas won't get any moisture to them unless you have a leak in which case the boards will fail anyway. i always use a backer board on studwork in wet areas, and use something like marmox/wedi board as they are a lot easier and quicker to fit than the likes of hardie boards.

another thing which a lot of people don't know about moisture resistant plasterboard is that if it is going to be skimmed your meant to green grit it first.
 
Soldato
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I kind of wish I’d gone for tile boards now but it’s all bought and they’re up now. I will be tanking the wet areas. To be fair some of the walls are half tiled half painted, only the wet area will be full tiled.

trying to decide if I board the back wall but I think I’m going to go for bonding then plaster as I’m a bit reluctant to just stick plasterboard straight to the wall. Not sure if I’m right to be though! We did it on one wall in the kitchen and no issues as yet, but not as wet a room as a very small bathroom.
 
Associate
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just be carefull on the weight of the tiles, if they are porcelain it will be borderline to much for a plastered wall(and definatly to much if boards are stuck over old plaster)
 
Soldato
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just be carefull on the weight of the tiles, if they are porcelain it will be borderline to much for a plastered wall(and definatly to much if boards are stuck over old plaster)

That back wall will actually have hardly any tiles on, it’s only up the window ledge and there’s a vanity/toilet unit and the bath, so I’m not concerned about the weight. Main worry is sorting out that mess at the top! I think it’ll be ok once I get started.
 
Soldato
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It doesn't matter if the entire wall is tiled or not really, you are limited on the weight you can put per square metre. Stick to ceramics and you'll be fine.
 
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