Skim Cracking Despite Thorough Prep

[Tozzy]

Hi,

I've just had a bash at skimming one of my bedroom walls and I'm very happy with the result despite being a bit of a first timer, except for the fact it has cracked quite badly in some parts. I'm not too concerned as I can fix it later with poly filler and the wall should look as good as new, but I would like to determine the cause of the cracks for next time as I've got another 3 walls to do. The undercoat is hardwall which I applied before Christmas which I sealed 2 times with PVA 1:5 (one part PVA to five parts water) exact ratios as I measured them. No problems with the hardwall as far I know, no cracks or anything and was applied at a consistent thickness. I mixed up my skim tonight and applied to the wall keeping the 1st coat as tight as possible. Then applied the 2nd coat at 2mm, and the 1st had already gone off which I assume is normal when the 2 coat is to be applied. Just before I got the end, that's when the cracks started to appear. Some small and some are making the plaster come slightly away from the wall.
Just can't think what could have caused it. Any ideas? Temperature of the room was pretty average incase that helps and my plaster mix wasn't too wet nor stiff if that should make any difference I don't know.

 

[ree]

Your troubles could have started before even the hardwall was applied - why was hardwall used?

Suction can be taken out of a very dry background by using water or PVA.

Your PVA mix was very weak - often PVA is mixed for suction at 3:1 or 50/50.

Skim is applied to the second coat of tacky PVA, and the second coat of skim is typically applied a few minutes after the first coat - there's no waiting for anything to "go off".

From what i can gather from your post, the whole lot might have to come off back to brickwork. That skim will never settle, and it will continue to blow and crack.

 

[Micilin]

How long did you wait after sealing the wall with PVA to skim it?

Did it suck at all when you put the first coat on, or pull in quickly?

If you did not allow 24 hrs for the pva to set, that might be your answer

 

[Tozzy]

Hi ree: No, I must have done the hardwall correct because Stevethespreader told me how to do it here:

//www.diynot.com/diy/threads/internal-blockwork-pva-or-just-dampen-it-down.419487/#3256789

He said to dampen down which I did (infact with copious amounts of water), and I had no problems when it dried. Then I let that fully dry out so then i had to PVA it 5:1 (5 parts water to 1 part pva) and I let it dry. Then i repeated, so it had a good two coats of PVA. Then after the PVA was dry 2 weeks later, I mixed up my skim and plastered as normal.

Your PVA mix was very weak - often PVA is mixed for suction at 3:1 or 50/50.

Yeah i think this may be the case, but I would also like confirmation from someone else.

there's no waiting for anything to "go off".

I disagree - With multifinish you typically get 20 mins to get your first coat on. Then you go back to the start and check whether the 1st coat has gone off, if so you apply 2nd coat a little bit thicker. If it's still as wet as when it was applied (depending on the suction of the background), then wait a couple of mins until it has gone off.

From what i can gather from your post, the whole lot might have to come off back to brickwork. That skim will never settle, and it will continue to blow and crack.

You are kidding me , I'm not ripping out fresh hadwall when that wasn't the root cause. I've skimmed a sand and cement wall before and dampened down as i went along and none of that cracked, so I'm guessing if I'd have not bothered with the PVA and just dampened down, it probably wouldn't have happened? What do you reckon people?

Hi Mcmillan:

How long did you wait after sealing the wall with PVA to skim it?

2 weeks. Surely that wouldn't be a problem?

Did it suck at all when you put the first coat on, or pull in quickly?

Yeah unfortunately, when i went back to the start to put on 2nd coat, the 1st was practically dry. That's when I knerw something wasn't right

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[stevethespreader]

you dont let hardwall dry out you skim it the same day or very next morning while damp and make sure it has a key either off the rule or devil scratch , blimey i remember a site i went to work on and there was miles of dried out hardwall where the poles thought they got paid just for floating and not skimming so i had the task of skimming it the hardwall was about 2 months old or dry, dry hardwall is one of the hardest surfaces to prep as its sucks like a good un
"edit" ive just seen ur pics and looks like u had loads of suction on the wall to cause that "like i thought" scrape the skim off and prep hardwall properly and reskim

 

[Micilin]

As per Steve, but I would always use SBR or Bondit for high suction background if you can't soak it enough. PVA not always the best, as you have found, even after leaving it for more than 24 hrs it didn't do the job.

 

[Tozzy]

Stevethespreader, thanks for your reply mate. I had no choice but to leave it because Chrismas was just days off and there was no way I could fit the skimming into my schedule. I really didn't think it would be that much of a problem if PVAed. Well, I'll def know for next time then. It did have a good key btw as it was rough as the rule came off it. So, I just need to be ultra clear now, I'll scrape all that skim off and get it back to how it was and then what's best way to prep the wall? 5 Water to 1 PVA or 3 PVA :1 Water ?? Also, how many coats do you reckon? And I don't do any damping down at all after that no? Thanks!

 

[stevethespreader]

as i said dry hardwall really sucks ,on that job we was on we had miles of old dry hardwall to skim so we used a hose for about half of it till it hardly sucked then they banned us using the hose because they dident want the floor wet so we resorted to shaggy roller on a pole dipped in water a couple of other walls were done with pva but best results were water lots of it

 

[Tozzy]

Yeah well, I think I'm going to have to do the same then, just use loads of water and then skim immediately after it. Seemed to work for me last time on sand and cement render.

Biggest problem I face now is getting all that skim off because it's actually not coming off that easy. I'm actually wondering now if I can just get away with removing just the defective loose parts and patching and sanding because to be honest it will have to be sanded in some parts even if I was to do it all again, because I'm a beginner it's like my 3rd attempt so I'm still going not going to be able to avoid small filling and sanding

 

[Tozzy]

EDIT: Definitely not going to get all of it off it would take me a week at least. The cracks are there (in numerous parts), but most are not bad enough to make the plaster loose so it's going to have a be a make good patching job.
Sorry for all the questions, but any ideas on the best way to patch? Shall I just use multi finish?? The whole wall was to be painted so I'm hoping I'll get away with doing that.

 

[Micilin]

If hosing down, I'd recommend hosing the day before as well as before skimming. It will help kill it.

 

[stevethespreader]

After looking at ur wall again I recommend to take it off

 

[stevethespreader]

After looking at ur wall again I recommend to take it off

crazing is reallybad

 

[Tozzy]

Moderator feel free to move this to the DIY Disasters section
Seriously though, I'm gutted now. The thought of having to take all that off with scraper and club hammer *sigh* Just pray it doesn't happen the 2nd time.
Well you live and you learn.
Thanks all for help. Good thing I know now what caused it.

 

[roy c]

Have you got room enough to dot and dab over the wall? You could put lots of small dots on it and dab some 9.5 mm boards over the lot .You would probably get it out to less than an inch, then skim as normal!!