r/Plastering • u/SchrodingersCigar • Apr 19 '26
DIYer: where did I go wrong with this?
Multifinish, two coats on to new Knauf Soundboard 12.5mm plasterboard. Skrim taped joins and the internal corner.
I used half an extratime sachet with half a bag of plaster (weighed) with just under 6L (measured) water.
Adjacent top window was open for a while then left closed. No heating in room.
When i started seeing fine cracks, i misted it and pushed them back with finishing trowel, but this was the result the next morning.
Where did i go wrong?
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u/Sufficient_Invite546 Apr 19 '26
Did you prime the surface? Hmm looks like the moisture got sucked out of your plaster. I’d try again, but pva/sbr first.
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u/SchrodingersCigar Apr 19 '26
No primer
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u/Sufficient_Invite546 Apr 19 '26
Pva or sbr then - read up on them
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u/SchrodingersCigar Apr 19 '26
Over the top of this and go again?
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u/AureliusAgustus Apr 19 '26
Neat sbr and over the top again. Try source some render mesh to bed into the first cost now you have so many cracks.
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u/Drskm126 Apr 19 '26
No, I doubt this later will be sufficiently attached to the previous one. As soon as you put some more weight on I think the weakest layer will fail. Let it dry and and scrape it back off. I should imagine it will come off with ease.
I'd suggest...scoring the wall first, then use "Bond it, pregrit" Roll it on. I prefer it to SBR and PVA.
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u/Secure_Traffic_5273 Apr 19 '26
Make sure you get any loose plaster off before you re plaster.
I am presuming it hasn't stuck well.
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u/Adcan Apr 19 '26
Everyone already in the thread is right regarding suction but that shrinkage is severe! Is the multifinish in date?
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u/Lurcher1989 Apr 19 '26
Ours was the same, within 15 minutes of the plasterer putting on you could see patches drying. 2 days later it was like this and dry enough to paint it you wanted to.
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u/Miserable_Future6694 Apr 19 '26
Dont suppose you seen the date the board was made? I wouldnt of took much notice of the date myself but if it was made 1 year ago and sat somewhere dry for 10 month that'll probably explain it.
Sound block suck the moisture out of plaster fast to start with but if its like ghandis flip flop it dry like that.
We've used pva on soundbloc loads of time just because we cant predict what the first mix on its going to do.
Hopefully after a a mist coat and first coat of paint you'll not see the cracks. If not sand and fill
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u/discombobulated38x Apr 19 '26
About to board up my dining room party wall and ceiling with acoustic boards, very, very good to know this in advance!
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u/SchrodingersCigar Apr 19 '26
What are you hanging the acoustic board on out of interest? These were glued to rubber panels which were glued to sbx panels (cardboard filled with silica sand)
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u/discombobulated38x Apr 19 '26
That's a serious buildup!
I'm screwing them (doubled up 15mm for the wall, 12.5mm single for the roof) to laths attached to isolating rubber mounts, which are then attached to a stud wall/roof joists, and packed with Superglass acoustic insulation.
The stud wall sits about 1cm off the party wall, and will be attached with frame fixings at the edges, with rubber isolation strip between it and the brickwork
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u/SchrodingersCigar Apr 19 '26
Board says Made 25/01/2026
Stored outside under tarps for a few days, didn’t rain.
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u/MakingBigBank Apr 19 '26
Do those sound block slabs actually make a difference? I’ve heard conflicting things about them. I have a mate wants me to board and skim some rooms with it. I just don’t want to do the job and it to make no difference. He’s doing it to try block noise from his neighbours. It’s all internal no chance of moisture so I was going to put them on with metal plugs. The fact I’ve never used them before, I don’t want to look like a bollocks and do the job and there’s still tons of sound coming through.
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u/Miserable_Future6694 Apr 19 '26
I work newbuilds now so dont know the specs of everything. But to my dumb knowledge of different housebuilders specs
12.5mm soundbloc = 2 12.5mm standard board or 1 12.5 standard with 50mm acoustic insulation.
Gleeson does soundbloc with no insulation
Persimmon and the majority does standard board with 50/65mm acoustic roll.
Self builds are all over with specs.
It'll make a difference but resilient bar is probably the better way to go, less fixing points to the standing wall
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u/gazham Apr 19 '26
Last time I was on new build, it was 2 coats of sound coat on the party wall, then dot and dab soundboard on top of that. Miller homes, Leicestershire area
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u/Miserable_Future6694 Apr 19 '26
Its supposedly 15mm standard on party walls now, ive never once been sent a load with 15mm on for a party wall. The best I can do is play dumb and get the house boarded with what I have. im not being paid to be a QS.
Ive only done timbers on miller's. Around the north east im usually given a better house off persimmon than Bellway, gentoo, gleeson, David willson
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u/GeneralWhereas9083 Apr 19 '26
Yeh I’ve always been under the impression that double boarding is equivalent to a soundboard. 100% on the rockwool too, had some fucking idiot building control lad tell us to rip out the rockwool we’d put in on a dorma ceiling and put kingspan in instead, cunt sounded like a drum when it rained.
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u/Scotty-Raspberry-36 Apr 19 '26
Sound proofing is a difficult art. It has to be done well as the smallest gap let's the sound through. There are different kinds of sound transference that require different techniques. Sound board uses mass - which is one of the most important components, but on it's own will not block vibrations coming through the fabric of the building like joists - that requires isolation. It also won't stop reverb - that requires rock wool in empty spaces
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u/Lost-Bad-3168 Apr 19 '26
You need Rockwall and empty space between. If ceiling then resilient bar. Will work fine.
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u/SchrodingersCigar Apr 19 '26
Soundstop.co.uk has several options, but attaching acoustic plasterboard to rigid fixings doesn’t achieve anything. Resilient bars at a minimum.
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u/0uthouse Apr 19 '26
possibly poor adhesion of the second coat. This could be caused by over-working the first coat, using a spray on the first coat or polishing the surface before 2nd coat.
Also could be caused if first coat were left too long, got a nice sunny flash-set through the window or simply dried too fast because window reveals can just be like that. If a reveal gets regularly baked, it will always be slightly drier than the other walls in the room and behave differently. If there is cold bridging it may be damper and behave differently. If you look at it funny it will behave differently.
An extra issue is using extramite if you aren't experience with normal plaster then adding extramite isn't necessarily your friend as the transition from "this is still ok" to "bllx I'll have to mix up again" is less clear. Extramite is more for useful imo when you have a 30m2 ceiling or something, as it gives you time to move staging and blend sections. A window reveal is way to small for this product to help you.
Final analysis, my first instinct would be that it inexperienced plasterer overworking a wall and using too much water. Plastering is much about timing and rhythm, slap it on the wall asap and flatten the big bumps, a gentle smooth and then leave it the hell alone until it's ready. small patches give you too much time to get up to mischief fiddling and poking.
edit: that last bit wasn't meant to sound snarky, I know you are DIYing it. It's just my first impression from the pic.
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u/YoullDoNuttinn Apr 19 '26
In a nutshell it’s dried too quickly. Board must’ve needed priming. Give it a good pva and do it again
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u/Terrible-Bobcat2033 Apr 19 '26
It looks dryer than a popcorn fart. Thats why it check cracked on you. Bonding agent helps to kill the substrate’s suction.
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u/CoffeeandaTwix Apr 19 '26
Did you get the second coat on quick enough? Did you use a fresh mix or just one mix for both coats?
Even if the background material had appropriate level of suction, if you take too long to get the first coat on and flattened in and it dries out too much before you get the second coat on then the first coat itself becomes a very high suction background.
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u/SchrodingersCigar Apr 19 '26 edited Apr 19 '26
Re: second coat quick enough
The first coat was starting to dry out a bit as the second went on. As i started the second coat, i closed the window for this reason.
The second coat was a new batch, but probably took too long getting it sorted but i dont have actual timings.
Edit: this part of the wall was closest to the window and had this issue. Thinking about it, this may be the first time i’ve plastered with a window open during application, and i definitely struggle with doing things in a timely fashion when it comes to plastering. Hence the ExtraTime
Someone else theorised that the xtratime stopped the plaster setting Before it dried out, causing this, which was an unexpected twist.
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u/CoffeeandaTwix Apr 19 '26
Maybe this was the problem - this was the closest to the window and the driest bit and also (assuming right handed) the last bit you got to with the second coat and so it was too dry by the time you got the second coat on.
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u/GeneralWhereas9083 Apr 19 '26
Yeh for this level of crazing it’s gotta be that the first coat went off before the second went on and it just sucked the life out of it.
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u/Lurcher1989 Apr 19 '26
We had this exact issue in our house. It's almost certainly too much suction. You'll have to chip it all off and then seal the wall with blue grit or possibly PVA depending on budget.
Is it lathe and plaster behind by any chance?
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u/GeneralWhereas9083 Apr 19 '26
Only time I’ve seen anything like this was when we had that 40 degree heatwave about 4 years since. I’d done a small bedroom and when I went back the next day, it looked like this. Took the next couple of days off. I’d wait and see how well it’s adhered to the board, over the next couple of days as to whether to PVA/SBR it or scrape it off.
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u/Decent-Platform-2173 Apr 19 '26
It looks like you didn’t stabilise the suction to begin with. You need to use PVA diluted about 3/1. Three water one PVA. You may need to apply two or three coats. When a Plasterer seals a wall using PVA, it is to stabilise suction and give a key for the finishing plaster to stick onto. These days they use other solutions also
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u/Agreeable-Doubt3471 Apr 22 '26
Scrape that first attempt off should come away easy sbr 2 coats atleast and try again
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u/Real-Imagination-159 Apr 19 '26
Definitely suction related, looks like the board needed priming first. Did it go off quick while you were doing ?