Plaster - Fault Diagnosis Required

[nmartin]

I am trying to repair a 3m x 1m area of 9" solid brick wall where the original render and plaster had 'blown'.

After cleaning and prepping the wall with 5:1 PVA solution, I applied 2 coats of sand, cement and lime render (5:1:1) - a scratch coat (approx. 5mm) and a top coat of 12mm. I left one week between each coat to make sure the render had dried through thoroughly.

One week later, I coated the render with a 3:1 PVA mix and then applied 2 coats (1mm and 2mm) of pink thistle plaster - approximately 20 minutes apart. A the end end of this, the wall looked pretty good - even if I do say so myself.

Unfortunately, my pride was short lived as once the plaster dried out it 'crazed' with hundreds of small, hairline cracks.

Plaster Problem

• nmartin
• 3 Mar 2013
• 1

Hairline cracks in new plaster

I did not apply any direct heat to the wall, but the house was warm (< 70 degrees).

Can anyone tell me what the problem is and where I went wrong?

Thanks.

 

[joe-90]

Well to start with you don't want it to dry out - that is the problem. Your substrate has sucked the water out of the plaster and left it high and dry - hence the cracking. From this point on you need to PVA the night before to seal the substrate - then do as you did this time.

 

[1john]

Sadly your plaster wouldnt have done this had you applied it as soon as the render was set, check what you have there is firm or else scrape it off and do as Joe says, PVA the night before then pva before you skim making sure your first coat goes on whilst still tacky.

 

[nmartin]

Thank you both for your help and advice. I will give it another go - this time with a more focussed attempt and priming the wall with PVA!

 

[jrplastering]

Well to start with you don't want it to dry out - that is the problem. Your substrate has sucked the water out of the plaster and left it high and dry - hence the cracking. From this point on you need to PVA the night before to seal the substrate - then do as you did this time.

yes you do sand and cement floating should be left at least a week before skimming if you skim it too early the float coat will shrink back and away from the skim coat

all hes done wrong is not killed the suction theres not even any need for pva just a spalsh of water will do ya

 

[joe-90]

It might in your world.

 

[jrplastering]

no not in my world, in the real world of being a proper plasterer you leave sand and cement for at least a week before skimming AS IT SHRINKS thats the reality of it, if you want to float and set straight away then use hardwall float all morning turn back and finish in after noon.
if you cnt cope with sand and cement without pva then you should not be plastering (unless your a newbie of course)

THAT IS HOW IT IS

 

[jrplastering]

also sand and cement cures not dries

 

[joe-90]

Well if it works for you then you keep it up.

 

[jrplastering]

i dnt really care if you want to keep doin it wrong but ur giving out advice thats incorrect and could get someone into alot of trouble