internal sand/cement with lime.

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beader said:
I used to work for a damp coursing firm years ago doing council houses, so we had to be in and out as they were usually occupied and as result were told to float up then skim the next day and i can tell you we had no end of problems with cracks etc .BUT i was young and easily led in those days and now i would never skim the next day but would leave it as long as possible (3-5 days). over the years have been to many jobs where the skim on a wall has been hollow and found it shelled off clean once you get behind it but the render has been fine, but this can also be because there was not enough key when rubbed up.
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i would say the majority of render /skim failures i have see, have been down to lack of mechanical/key due to poor rubbing -up , also the incorrect skim plaster for S&C backings ........
 
johniosaif said:
I have seen it done by damp firms, even seen them putting in bonding in the render to speeed it up, damp proofers are the cowboys of our game generally speaking ie those that only do a metre high all their lives..
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Better to put rapid cement in to speed things up....That way you can skim the same day..........:RpS_wink:
 
gooner59 said:
i would say the majority of render /skim failures i have see, have been down to lack of mechanical/key due to poor rubbing -up , also the incorrect skim plaster for S&C backings ........
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What is the correct skim plaster???? Whats wrong with plane old Board finish?????
 
carlos said:
What is the correct skim plaster???? Whats wrong with plane old Board finish?????
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multi is correct nowadays for skimming over render, board is just for board, carlite for skimming only carlite products, used to be siraphite b for render but muti replaced this and as its name suggests.....
 
im with andy on this one the job im on at the mo they are in a hurry to finish before christmas but we have just floated out the downstairs level and told them were off for at least 4 days to let it cure or it will all be on the floor and i have seen this happen in abundance when same as beader i used to sub off a damp firm and they would float monday then get me in to skim it the next day couple weeks later there would be cracks that a family of albanians could live in a large sheets of skim on the floor and i would then get paid again to sort it out, with a great big I TOLD YOU SO to the main subby haha
 
also another area that has not been explored is ettringite failures ,between cement/gypsum ???? the cement reacts against the gypsum and forms crystal's that push the skim coat away from the render and de-bond !!??
 
And yet another case of someone being told the correct methods, not liking the answer and then arguing the toss. Seems to be a growing trend on here.
This thread, I'm out.
 
kebab king said:
Malc it be old school, but imho its still the best way, and a builder i do bits for only floats with sand and cement, and it always comes out mint, if left to cure properly.
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i think that sand and cement floating is obsolete. when i see the key on modern blocks ie celcon, celcon solar blocks, i have seen sand and cement sheet off these blocks.
there was a time a few years back that some plasterers did not devil float but just floated smooth as they thought that multi finish had improved adhesion, so why did you need a load of scratch lines slowing you finishing down. i have seen the set on these walls breaking key.
if ever a builder asked me use sand and cement floating i always ask is he supplying a fork lift to get the material up above ground floor? he then realize that bagged plaster will be cheaper then to hire a fork lift and driver
 
malc said:
i think that sand and cement floating is obsolete. when i see the key on modern blocks ie celcon, celcon solar blocks, i have seen sand and cement sheet off these blocks.
there was a time a few years back that some plasterers did not devil float but just floated smooth as they thought that multi finish had improved adhesion, so why did you need a load of scratch lines slowing you finishing down. i have seen the set on these walls breaking key.
if ever a builder asked me use sand and cement floating i always ask is he supplying a fork lift to get the material up above ground floor? he then realize that bagged plaster will be cheaper then to hire a fork lift and driver
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Sand/cement suits refurb work, especially old wrecked and damp properties, light weight plasters are very expensive still compared to sand and cement
 
johniosaif said:
Sand/cement suits refurb work, especially old wrecked and damp properties, light weight plasters are very expensive still compared to sand and cement
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we do a lot of refurb work, it is years since we floated in sand and cement. damp work is either thistle drycoat or limelite. usually hardwall and bonding in other areas,
but the one i am doing at the moment is in chalk with hair in it , i am not enjoying it ,it is ready mixed with too much hair in the mix for my likeing making it hard to spread, on wood lath and reed ceilings .
the chalk materials for a 20m square ceiling cost £400
 
beader said:
You can't skim sand n cement the same day as the surface will not be firm enough to take it as the grains of sand come away (if that makes sence) and the skim just falls off ,its a disaster to even try it.
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i must admit i have done this a couple of times many years ago ,and it really pulls the finish in,sucks like hell .its called a green suction if its set the same or next day,but your right its not the done thing.
 
This is a age old topic. Been done over and over. It should be left to cure but not everyone has the time.

I have worked on the books for DPC firms in the past and most expect anything less than 15-20m2 floated and set the same day in my experience. It can been done with rapid cement, hot water, putting fan heaters on the wall, but I wouldn't want to put my name to it.

When I was doing this work 6 out of 10 jobs had some kind or come back. All plastering related.
 
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johniosaif said:
I disagree strongly on this Bubbles. never had it debonding in 25yrs of plastering, more likely debonds due to dust if left too long..
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Fair play if you've done it and it works for you, but I've never cos I was always told it would lead to system failure :RpS_thumbup:
 
del said:
What happens if you let it dry completely, say a month, then skim it, and sucks like fcuk ?
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It depends on the sand used and more importantly the substrate you're working over.
Good clean (ish) sand over Celcons/Thermalites you'll have no problems with suction at all. I once floated out a house and didn't get to set it for three months. The only bits that needed dampening were the Catnic lintel heads.
If you're working onto low suction blocks then IMO it is even more important to make sure that the S&C has cured but you'll get a lot more suction that when working on high suction blocks.
 
No I meant, how does he explain why he gets more suction when skimming render on low suction backgrounds, than Celcons.
 
DannyMac said:
No I meant, how does he explain why he gets more suction when skimming render on low suction backgrounds, than Celcons.
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I think it's because on Celcons the water is drawn into the block and then slowly let back out through the muck helping it cure properly, whereas on low suction backgrounds the water comes out into the atmosphere thus not letting the muck cure properly.
Exactly the same as when outside rendering, you always need to wet down the first coat more on the low suction backgrounds to get it to cure, don't you?
 
lucius said:
Plasterboard dosent have a key.
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plasterboards have microsophic holes in the paper that the skim attachs to i.e if you stand on the a lot with dusty boats or sumert or if you use mouisture boards which have a silicon coat over then you need to pva to provide a key as the holes will have been filled in
 
DannyMac said:
No I meant, how does he explain why he gets more suction when skimming render on low suction backgrounds, than Celcons.
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i was taught that you get oppoisite suctions , low suction on floating =high suction on set. high suction on floating = low suction on set.
 
malc said:
i was taught that you get oppoisite suctions , low suction on floating =high suction on set. high suction on floating = low suction on set.
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I didn't know that. i haven't skimmed much S&C TBH.

Why do you think it works that way?
 
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