Skimming 'blown' off sand/cement backing?

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I did 2 small walls in sand/cement [about 4:1 mix] for family (brother). Now I didn't mix, he got his lad to, but he's mixed before and it's not exactly brain surgery.

It was a cold, windy February day but nowhere near icy and I'm pretty sure I skimmed the backing coat next day

A few weeks later he tells me the walls have a few cracks in them and the skimming has 'blown' away from the wall. When I looked, you can get your fingernail behind the finish and pull it off in quite big pieces (3, 4, 6 inch pieces). On the wall by the front door, it was all blown off the wall in one complete sheet. When I looked yesterday there's like a 7mm gap between finish and backing.

Around the same time I did a job for his other son of 2 sand/cement walls in a bathroom, again, this householder mixed. The brickwork was bad anyway so iirc, I asked him for a weaker mix which may have been 4, 4&half or 5 to one with waterproofer to kill the suction as there was delay between when I could access them again. [I would have dot & dabbed them but 'he thought' the S/C may add strength to the wall]

These walls both cracked looking like 'crazy paving'. I scrimmed the cracks and skimmed walls. 3 or so months later I'm patching up the inevitable bits when I notice a bit of the window wall skimming has blown off. I pull the loose off and patch it. As I'm trowelling it up neighbouring areas start to come away and the same happens to other areas next to the holes I'm patching. Before long, it's like 60% of the wall has blown off.

After a bit of head scratching, I told him it must be temperature based and his walls must have froze the night I skimmed them. Freezing water = expansion = it has pushed itself off the backing coat. [I'm not convinced with this myself but I'm grasping for an answer]

But seeing that the second job yesterday has also gone the same way, I'm looking for the common denominator and it could be the sand. I recall the sand (red builders in 1 ton sacks) was brought from 1st job to 2nd. Now he had walls built around his property and the mortar shows salt effervescence on the joint and, iirc, on the brick.

When I look at the back of the blown skimming, it's like a cast of the backing coat along with rough texture and the nail lines from the float. They key was there, the adhesion wasn't.

I don't think the 'febmix' was the same in either case and beyond that I'm stumped. The common denominator to both jobs was me and the sand used. I've had walls crack before (mostly due to suction issues) but not this blows off completely.

Any ideas out there?
 

algeeman

It’s A Boy
Try the roulette table or sports book.
Sometimes it works but most often it dont....
Hope this helps mate.
 

Fatarm

Trainee mod
How long did you leave the s&c before skimming?
sorry just noticed you skimmed next day...that may be the prob?
 
I have done nothing different than how I've worked thousands of times already. My thoughts take me to contaminated sand, either not washed properly (re: the salt) or whether we introduced something. The wall by the front door (images 1 & 5) is 3ft wide, 9 high and with the original dado re-bedded back on to the brickwork at the same time I skimmed. The dado is solid but above that the whole skimmed surface has come away in one big sheet.

Nos 2, 3 & 4 show the upstairs wall. No2 shows brother fixing architrave half way thru skimming, the other 2 show the cracks and how the skim falls away. This was after a brief examination with the carpets down, etc. I'm sure I could have pulled a lot more off.

Also, can you see the water stains in Nos 3 &4? I don't know if that is connected.

Also, as this has happened, now we don't know how to rectify it. I'm thinking of dot & dabbing board on top of the floating 'to be 100% sure' as it'll get round the crack and blown skimming issue.




Photo0259.jpg Photo0261.jpg Photo0281.jpg Photo0282.jpg Photo0260.jpg
 

SpankySouthport

Private Member
Yeah as bobby said should have let it cure before skimming then would have shrunk back and cracked before the skim..

always this risk skimming next day although a lot still do..

One of the pics looks like there isn't much of a key

always keep an eye on the mixing and don't skim next day I will certainly learn from that

Pull the finish off and reskim should be fine good luck with it :-0
 

raggles

Private Member
Did you devil float that S&C cos i cannot see any marks from a devil float or is it just me :RpS_confused:
 

jamesthefirst

Private Member
Roll on a weak PvA and let that dry followed by a coat of Betakontakt or Microgobetis 300 and reskim with some fibres from Source4me
 

zombie

Private Member
Yeah as bobby said should have let it cure before skimming then would have shrunk back and cracked before the skim..

always this risk skimming next day although a lot still do..

One of the pics looks like there isn't much of a key


always keep an eye on the mixing and don't skim next day I will certainly learn from that

Pull the finish off and reskim should be fine good luck with it :-0

bang on brother!:RpS_thumbup:
 

tam

Active Member
Wouldn't use waterproofer in S&C when skimming over it. You need suction to help the bond.
 

3kids1dog

Member
Your mixing too strong mate . We mix our sand and cement 8:1:1 for internal work are you devilling it up enough? Are you you letting your backing dry and shrink enough before you skim?
 

TheWand

Active Member
I would just like to echo what some of the others have said

 
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