Scratch coat render

[GlenT]

I am a plasterer but havnt done much rendering.I contacted celcon with regards to rendering their blocks and they told me to render 6:1 wich i know is weak but what they said made sense. "the mortar shouldnt be stronger than the material". This also applies to brick work. I wetted the wall applied the render and then wetted the render during drying process but after 4 days i can scratch the render off with my fingers. If i then went over this with a topcoat would it be ok or does it all need to come off and ignore what celcon have said and use a stronger scratch coat? any help would be appreciated

 

[stevethespreader]

I am a plasterer but havnt done much rendering.I contacted celcon with regards to rendering their blocks and they told me to render 6:1 wich i know is weak but what they said made sense. "the mortar shouldnt be stronger than the material". This also applies to brick work. I wetted the wall applied the render and then wetted the render during drying process but after 4 days i can scratch the render off with my fingers. If i then went over this with a topcoat would it be ok or does it all need to come off and ignore what celcon have said and use a stronger scratch coat? any help would be appreciated

can i ask what sand did you use? if you used just ordinary soft or builders sand maybe it wasn't coarse enough

 

[GlenT]

Yes i used a bulk bag of rendering sand from selco.Ive only put the scratch coat on 1 wall at the moment,which was done just incase something went wrong. I was told this afternoon that it will be ok when the topcoat goes on. I have rendered a small patch to see how it goes.I have hosed the scratch down and the render stays on,comes off if you rub it

 

[Micilin]

6:1 is weak, regardless of what anyone says, it will be easy to knock .

Steve's asked about the sand

Did you over mix it?

Put too much plas in?

Hot day?

 

[GlenT]

I did let it turnover in the mixer for a while because i was also told thats the best way. Not to have it too wet as there is more chance of shrinkage when setting hich will then cause cracking. I used waterproofing and i used a measuring jug to make sure i didnt overdose. It wasnt a hot day and i dampened the wall as it was drying as told by celcon.Waiting to see what the patch goes like once its cured. I was on site a few years ago and the renderer had the same problem using4:1. The only difference was that his was reall powdery and would probably have come off with a hose pipe. Mine doesnt come off with a hose pipe even at high pressure only when rubbing it . The renderer scraped his off, i dont know what hapened after that.

 

[GlenT]

I wondered if SBR on the blockwork first would help?

 

[stevethespreader]

im a bit like you in a way glen, im a plasterer but don't get much chance to do rendering, i have more knowlage of it than i have experience, i would never have given it a 6-1 scratch coat , remember that your float coat has to be weaker than your scratch coat, so now your looking at a float coat of 7-1 which is even weaker than the 6-1 celcon recommended, if your going to give it a 6-1 float coat the scratch coat should have been 5-1

 

[GlenT]

I agree with you steve. Its just that i know a few people whos render has failed on blockwork so i thought i would email celcon for their advice. I have emailed them again to see what they have to say. No doubt they will find some way round it . Did you damp it down while it was drying? yes. Oh you must of wet it too much then. As i said previously it does make sense that the mortar shouldnt be stronger than the block.When laying blocks you should also use 6:1. Thanks for your input though

 

[troweladdict]

this were you you went rong m8te you needed to PVA or SBR the wall first before you render it plus are the blocks smooth or are they keyed up ie dressing hammer or from the manifactorer.
your scrach coat needs too be a 4/1 mix.
top coat 5/1 mix.
wet the wall with a hose give it 5mins then a good drink of watered down PVA 4/1mix.
then hit it with a 4/1 mix building sand/soft sand plastering sand is ok
use a bit of water proofer in the scrach.
rule off filling dips/hallows flatter the better for finish coat but dont over work it.
use a render comb too scrach it up ready.
leave over night then a good drink of water agen then a good drink of PVA agen.
hit it with top coat rule off filling dips and hallows trowel it off leave it till it starts to go then use a jumbo yellow spung with a bucket of clean water.
rubing it sircles away from you filling dips with left over render.
rember to set beads before the scrach try useing plaster board addy with PVA use some pins just too hold in place. sorted if it helps m8te

 

[GlenT]

Thanks trowel addict. But i have allways been told not to use pva outside only sbr.Celcon say that a stronger mix will cause cracking but they seem to be trying to cover their arse on everything. They said to just wet the wall but not too much.

 

[stevethespreader]

Thanks trowel addict. But i have allways been told not to use pva outside only sbr.Celcon say that a stronger mix will cause cracking but they seem to be trying to cover their a**e on everything. They said to just wet the wall but not too much.

unibond do an external pva (eva) that is sutable for cemet based screed and renders i think addict was on about exteral pva, so what are you going to do glen ? put a 6-1 float on a 6-1 scratch ?or 7-1 float, or if its not that much you have put up, take the lot off and start from "scratch" scuse the pun lol

 

[Richard C]

Depending on the base thickness you’re aiming for, a render base is usually best laid on in two coats, a scratch coat followed by a float coat which should be a slightly weaker mix. I use (sand/cement/lime) 4:1:1 or 5:1:1 where heat is a factor, followed by a slightly weaker float coat. Assuming you used fresh cement, if it’s still so soft after 4 days something has clearly gone wrong; either the mix was a whole lot weaker than you thought, you overcooked it or the cement bond has suffered as a result of too much additive or water on the block base or while the render was setting. Finishing over a soft base will make it very weak & susceptible to damage, even assuming you have good bond in the first place.

 

[troweladdict]

7/1 render is ok on a play house now days it 8/1 mix steve iv never rendered in my life i just know how to do it
rember them pics i showed you m8te that was over concrete blocks rubed up sprayed fininsh white rendered a loads of celcon wall wall never no probs but stick to your guns m8te 7/1 finish coat stand back and look at it nice job if you need two plasters/renders m8te me and steve are looking for a bit of work