Eggshell rendering

Simon85

Well-Known Member
Seen some rubbing up, once its cured get that eggshell cracking look, wondering what causes it? It hasn't happened us, but a few jobs I've seen, actually got a firm used to labour too, seems to happen them quite often. I was thinking maybe the scratch coat too strong causes it? For I mind at the time couldn't believe how strong they wanted the scratch. Normally I mix scratch at 16 shovels to a bag,
They wanted it 2 bags to 25. Thought that was unusually strong, maybe too strong, so if any movement in the blocks then the scratch coat just shelled underneath? Anyone know what normally causes it?
 

Rigsby

TPF Special Forces
Too strong a mix could cause it as will putting the second coat on too soon before the scratch coat has cured.

I have had it when I have floated sand and cement flat but scoured then flattened the scouring smooth with a stainless trowel then a light sponge finish. Looks ok until it rains.
 

vfr12

MOTORC*NT
Too much lime in the top coat can also cause it to happen
Not really, quite the opposite.this effect is down to rapid drying. The key in rendering is to control suction. You don’t, you are f**k*d. That’s for every stage of the process. Always think suction.....suction.....suction.
 

Simon85

Well-Known Member
Too strong a mix could cause it as will putting the second coat on too soon before the scratch coat has cured.

I have had it when I have floated sand and cement flat but scoured then flattened the scouring smooth with a stainless trowel then a light sponge finish. Looks ok until it rains.

I know not letting scratch coat dry enough, you can sometimes see scratch coat through the rubbing up, had it recently a panel that took a lot of water down the corner the roof dripped off, could see the scratch coming through. We cut square out where could see the lines of scratch, through dryers onto it then coated and rubbed it in. It worked, but had it been a full panel it wouldn't have been aj ideal fix, to basically strip and redo. Is there any other way to save it if it starts that ?
 

Rigsby

TPF Special Forces
I know not letting scratch coat dry enough, you can sometimes see scratch coat through the rubbing up, had it recently a panel that took a lot of water down the corner the roof dripped off, could see the scratch coming through. We cut square out where could see the lines of scratch, through dryers onto it then coated and rubbed it in. It worked, but had it been a full panel it wouldn't have been aj ideal fix, to basically strip and redo. Is there any other way to save it if it starts that ?

I find that happens more in the cold months. Deep scratches are good but don’t help slumping matters.
 

solway

Active Member
I find you see crazing/shrinkage cracks more on south facing and west facing elevations possibly due to the sun and breezes warmer on these areas. Too much lime or liquid plasticiser/waterproofer can cause cracking but also not enough lime/liquid plasticiser/waterproofer can also cause crazing/shrinkage cracks. We did a house about 3 years ago. South and west elevations got crazing on them after a shower of rain but no scouring up was done. Floated up fine and sponged off with no premature drying while working on it. The north elevation is 100%,no problems rubbing up,no crazing or shrinkage cracks. The east elevation which is the front of the house pulled on on us a bit quick so we had to scour back up with our trowels and sprinkling of water and then sponge off. I was sure I would get crazing/shrinkage cracks but the wall is mint. Iv put it down to the fact that the sun never gets to it at all. Maybe a bit in the morning but no heat from it at this time. My personal opinion is that we don't keep our walls damp for 3/4 days after rubbing up but who is going to pay us for standing around spraying a list coat 10 times a day for 3 day's..... I know concrete installers spray a liquid curing compound over their concrete upon completion. If there was a product like this for rendering I would definetly buy some and use it(peace of mind) Some curing compounds van be used on render but then there's some problem if the walls need to be plastered then. I must look into it more. Rendering.....keeps me awake some nights....
 

vfr12

MOTORC*NT
As I said before suction is the main factor for me. Then you have to question your mix and the aggregate used in the top coat. Crazing is a sign of weak finish. Your mix might have been ok for the rest of the job but in the areas where you have the cracking it shows something has gone wrong . It’s defenitely the quick drying causing it and shrinking the top coat. Too fine aggregate in the mix can cause it as well . In this case the binder can’t fill all the space between the aggregate and the bond is not what it should be. Using lime in the s/c mix helps transport and fill as much as possible the gaps between the aggregate, plasticiser not so much, hence i prefer using lime in the mix. As said above We don’t get paid to nurse the walls so here is helpful trick in the sleeve -compress the top coat with float. That helps and reduces the shrinking and can eliminate the problem completely.
 

solway

Active Member
Elaborateore on a weak finish to you. Too much additive/admixture or too much sand like 7 1 1 or 8 1 1....
I agree with you about everything above. Iv kind of settled on 5/6 sand 1 cement 3/4 lime. All bucket gauged. With some waterproofer added to make it more workable. Also we use liquid plasticiser in scratch but will probably switch to waterproofer in scratch from this month on. You say compress with float. Do you mean 2 tight floats and then sponge off?
 

t33ch

Active Member
Seen some rubbing up, once its cured get that eggshell cracking look, ... Anyone know what normally causes it?

Seen a couple of jobs with fine cracking where I know they've used building sand (one ended up going legal).
Very fine aggregates screw the w/c ratio and weaken the mix.
 

taylorhendersonplastering

Well-Known Member
Not really, quite the opposite.this effect is down to rapid drying. The key in rendering is to control suction. You don’t, you are f**k*d. That’s for every stage of the process. Always think suction.....suction.....suction.
A knowledgeable man is worth his weight in gold..
Not many plasterers out there these days that know their stuff..well said my man...
Trouble is it’s like anything either you have got it or you don’t..
Any decent plasterer should understand the science behind the game..
 

vfr12

MOTORC*NT
A knowledgeable man is worth his weight in gold..
Not many plasterers out there these days that know their stuff..well said my man...
Trouble is it’s like anything either you have got it or you don’t..
Any decent plasterer should understand the science behind the game..
Old school here man:D
 

vfr12

MOTORC*NT
Elaborateore on a weak finish to you. Too much additive/admixture or too much sand like 7 1 1 or 8 1 1....
I agree with you about everything above. Iv kind of settled on 5/6 sand 1 cement 3/4 lime. All bucket gauged. With some waterproofer added to make it more workable. Also we use liquid plasticiser in scratch but will probably switch to waterproofer in scratch from this month on. You say compress with float. Do you mean 2 tight floats and then sponge off?
The science says multiply the amount of binder by 3 and you will get the correct ratio aggregate . For instance 1 lime/1cement will need 6 sand. My preferred mix is 4/1/1 and 5/1/1 , but knowing measurements depends on how accurate you are with the bucket or shovel, I can say anything near it . You can end up with the same strength for scratch and top coat, but that's not too bad really as long the scratch is weaker than the substrate you'll be fine.
If it's easier for you to rub it with the sponge that's fine, but finish it at the right time with the float. My favourite is the diamond one from refina- just glides over the render and compression is in place. You can tell the difference strait away and see how it pushes it back. No sponge after that, you'll only make it worse.
 
Top