Plaster base coat on old thermalite blocks

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Jamesgilmour

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Hi everyone ,

Just stripped some tiles off in my kitchen and most of the old render and plaster has come off leaving bare thermalite blocks. What is the best base coat to use prior to 2 coat multi finish skim. Was reading about Hardwall but everyone says its a nightmare due to high suction rates. Would 2 coats of PVA do the trick. Or am I better of just mixing sand and cement 4:1 and putting a traditional scratch coat on ?

Thanks !:RpS_thumbup:
 
Hardwall is the easiest option, a light coat ofweak pva 5/1 then a 3/1 coat water to pva,good to go with hardwall
 
People who say to use any pva to any backing coat don't know what there talking about , you can't form a film between the substrate and the plaster ! Just damp the blocks down with water
 
80nobby80 said:
People who say to use any pva to any backing coat don't know what there talking about , you can't form a film between the substrate and the plaster ! Just damp the blocks down with water
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I think you don't know what you just said... Old walls that are hacked off are perfect for pva or a pregrit, read and learn don't just blurt out...
 
johniosaif said:
I think you don't know what you just said... Old walls that are hacked off are perfect for pva or a pregrit, read and learn don't just blurt out...
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john, put your glasses on and read the instructions on the bag of hardwall.
 
You mean gyp- bond which is now proved to be failing where hardwall is now starting to blow and come off , so now if taught you something
 
80nobby80 said:
You mean gyp- bond which is now proved to be failing where hardwall is now starting to blow and come off , so now if taught you something
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Always happy to be wrong and learn,but I would still use a weak pva over old bricks prior to hardwall. Gypprimers are recommended by bg but I haven't used these ever...
 
Went to a job yesterday,plaster is bulging under the wallpaper in several places in a 15 year old garage conversion,its parted away from the block work and started to tear the paper in the worst areas.all four walls have at least one blown patch,will know more when I go back to take it off,just funny you have just mentioned this.
 
I havnt seen what float coat it is yet as its under the paper but as you said I suspected browning.
 
If you're putting on PVA or SBR, go on well thinned down. You need it to soak into the block, not form a film on top. That will hopefully control suction, next problem strength. The most basic principal of building is that your masonry unit is stronger than your mortar. 1:1:6, = 8N, blocks 2.8, 3.6 N, Problem.
Look up St. Astier Tradeco, it makes a fantastic render/plaster for lightweight blocks.
Failing that, carry some very thin wire, it's usually cracked blocks,when you get called back to cracked render on new blocks, if you can put 30mm of wire into a 20mm crack, then it's a little less like 'your problem'
 
the problem with hardwall or browning is because it's a powder base plaster it will only last 15 to 25 years , all depending on the substrate . Sand and cement all day long !!
 
80nobby80 said:
the problem with hardwall or browning is because it's a powder base plaster it will only last 15 to 25 years , all depending on the substrate . Sand and cement all day long !!
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A man after my own heart, I keep getting called dinasaur because of my fondness for sand and cement internally ,often use hardwall etc and ocr on occasion but prefer sand and cement for myown jobs,
 
80nobby80 said:
the problem with hardwall or browning is because it's a powder base plaster it will only last 15 to 25 years , all depending on the substrate . Sand and cement all day long !!
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where do you get the information that browning/ hardwall will only last for 15 to 25 years ?
the new style carlite [ now called thistle] browning, was first produced in 1964. we have used it on thousands of properties without failure.
thistle hardwall is a lot more recent product and i do not think it has been out for 25 years.
cement and sand went out with the ark and is not a recognized backing for british gypsum finish plasters.
 
johniosaif said:
I think you don't know what you just said... Old walls that are hacked off are perfect for pva or a pregrit, read and learn don't just blurt out...
Click to expand...
Totally agree here john in a way when you hack of old plaster its always a good idea to give a watered down coat of pva just for peace of mind if anything!:RpS_thumbup:
 
Hardwall is a cement BASE backing coat it will last no more than 25 years max ,there's no better backing coat that sand and cement, I dident say it's only been out for 25 years. And with bg plasters that's why it's called MULTI finish so it goes on a multi of different back rounds!
 
Actually it's gypsum based,(calcium sulphate)it only contains 3.5% cement and under 1% lime.just so you know:RpS_thumbup:
 
80nobby80 said:
Hardwall is a cement BASE backing coat it will last no more than 25 years max ,there's no better backing coat that sand and cement, I dident say it's only been out for 25 years. And with bg plasters that's why it's called MULTI finish so it goes on a multi of different back rounds!
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what rubbish! how can you state that it will only last 25 years when it has not been available for 25 years?
multi finish will finish bg backgrounds but it is not universal. sand and cement is NOT a recognized background for multi finish.
 
BG used to make a modified form of Browning for thermalite called 'Browning HSB' [High Suction Backgrounds]. Mustn't be enough demand for it nowadays seeing as dot&dab is king.

If cost of bagged background coat is an issue, or you have damp probs, go with sand/cement? I would damp the wall with a hose (lot of water) and give it a scratch coat, 4 or 5:1. The float coat would then be weaker @ about 5 or 6:1... but that is a goal... an ambition... what the labourer could bring anything up. :)

Or you could dot & dab it (plasterboard and skim it). That would get you away from the thermalite issue.... if it is an issue, never used to be!


The below is BGs official take on the subject:

Brickwork / blockwork
The surface must be clean, dry and suitable to receive gypsum plaster. Control suction with water if necessary. If suction is severe the background should be pre-treated with Thistle GypPrime.

On high suction brick / blockwork the use of Thistle Hardwall or Thistle Tough Coat is recommended.
 
malc said:
where do you get the information that browning/ hardwall will only last for 15 to 25 years ?
the new style carlite [ now called thistle] browning, was first produced in 1964. we have used it on thousands of properties without failure.
thistle hardwall is a lot more recent product and i do not think it has been out for 25 years.
cement and sand went out with the ark and is not a recognized backing for british gypsum finish plasters.
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I thought that BG recommend board finish for sand and cement .
 
Applications
Thistle Board Finish is designed for low-suction backgrounds
e.g. plasterboard, Glasroc F MULTIBOARD and Glasroc F FIRECASE,
Thistle Dri-Coat, sufficiently flat concrete and other flat
surfaces treated with bonding agents.
 
2coats pva first 5/1 then second 3/1 thin coat bonding scratch light pva 4/1 let go tacky then 2coat multi if your re tiling don't polish second coat....and good luck...
 
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