Trouble with internal sand and cement rendering

[vaugi]

Hi Guys,

I'm hoping someone can help, I'm having a real problem trying sand+cement plastering for the first time. I've done quite a bit of skim, for myself, not a pro but I guess I'm competent when I take my time.

I'm re-plastering after injecting with a DPC cream, where everything suggests sand+cement with waterproofer for the first coat.

The problem is the the plaster slumps a lot, and the water gravitates down make the bottom parts slack off and usually fall off. I'm also finding the consitency hard to work with moving it around and separating.

Here's what I've done, mostly gleaned off these forums. I'm hoping its something simple that I'm doing wrong.

- Wet down brickwork

- Put first coat on, 4:1 building sand cement with gauging water (1:30 Everbuild waterproofer to fresh water)

- I've been trying to get on about 7mm ish, but I need to build up about 25mm in some places so have done a few of these coats, scratching in between

- final coat of 5:1 without waterproofer

Unfortunately I had to leave it overnight before putting on the final coat (two coats in places due to depth). I think this might be my problem...I tried wetting down the previous days work but that didn't help, maybe made it worse.

Is it because the first days coats used waterproofer, and hence the final coat has very little suction? One these final coats I had to leave it overnight to dry, even after 4-5 hrs it was still very wet, dragging down to the bottom of the wall and dropping off.

I've tried altering the consistency of the mix (more/less water) but it doesn't seem to make much difference.

If anyone has any tips/pointer I'd be very grateful...even if it's broken down on every step for a beginner.

Cheers

 

[Alastairreid]

Did you scratch the 1st coat?

Did you put plasticiser in the 2nd coat.

 

[vaugi]

Scratched all coats (I plan to devil float final coat, but had to leave again today to dry out)

Is plasterciser the same as waterproofer? I put waterproofer in first 2 coats, but not on the final one I put on today.

 

[Alastairreid]

Add a plasticiser to the final coat! it helps to bind it, what a plasticiser does is introduce into the mix millions of air bubbles ..this is called entraining ..the bubbles make the mix softer and more pliable .

http://www.everbuild.co.uk/febmix-admix

 

[roughcaster]

First of all, how did you mix up the render, cement mixer or shovel, (hand batched). Also, was the waterproofer a plasticiser too,, (a combined waterproofer/plasticiser). A good render mix needs to be "aerated", which makes it lighter and fluffier to work with.
When you wet the wall, always let it drain off before you start applying the scratch coat and the top coat render. Never overload a wall, don't put render on too thick or it will "slump" which is probably what's happened with you, along with the mix being too "heavy, and the wall being slightly too wet to start with which makes the mix slide down the wall as you put it on.. The render top coat should again be well mixed and aerated, the scratched wall should wetted down and allowed to drain off. Start spreading on the render top coat, again, don't overload the wall, and don't forget the plasticiser. Lime in the mix is also of great benefit.

 

[Alastairreid]

Well said R/C

 

[vaugi]

Thanks both for the help.

I mixed up using paddle mixer, about a bag of sand a time (only need to do one wall). I have to say I read a few times about getting a creamy buttery mix but never seemed to be able to get that.

The waterproofer was just an integral waterproofer http://www.everbuild.co.uk/building-products/building-admixtures?product_id=131

When I first wet the walls (brick) there wasn't an excess as it soaked in pretty quickly. Is leaving coats overnight a bad thing (esp after using waterproofer)? When I wet the walls the next day I noticed the water didn't really soak in much and didn't run off easily. I tried doing one bay after wetting and one bay not wetting up at all, both the same problem.

I would probably say I was happy with how I was building it up the first day, slowly adding coats after leaving to dry a bit, with no slumping.

I was conscious about overloading the wall, definitely <10mm a time, more like 5-7mm really...as a say, it wasn't just slumping, it was the water seeping slowly down the wall and dripping off at the bottom.

Is there much difference to using plastering sand vs building sand I've been using? Is it just a case of being finer for a better float?

I'll get some plasticiser and give it a go... thanks again for the help.

 

[roughcaster]

I've never used a paddle mixer to mix up render. I've either used a small cement mixer, or hand batched using a shovel. It's surprising how good a mix you can get using a shovel but it's hard graft.
As Alastair said, it's minute air bubbles caused by the plasticiser that makes a mix workable. Water on it's own just makes the mix "wet and heavy".
Always use plastering sand for rendering. It's a slightly coarser/sharper sand that's been "washed", so no clay particles in it, You can also mix plastering sand and sharp sand together for rendering..
You'd be surprised how thick you can apply a well mixed batch of cement/render, with the correct additives/plasticiser in, without any slumping at all. It's all experience and practice at the end of the day, and the only way to learn is to ask questions, give it a go, and learn from your mistakes. We've all been there.

 

[roy c]

You said you mixed 4-1 "Building" sand well where I am building sand is only used for brick or block laying it is to fine for rendering . Like R,C. said you need sharp sand for render mix....

 

[vaugi]

Thanks roughcaster and roy C.

I'm not sure where I got it that I had to use building sand, that's the trouble with so many forum posts, I must have seen one of them advising on building sand.

I've got some plastering sand now and will give that a go with the plasterciser, hopefully get it sorted, looking forward to retrying, better than pulling my hair out the past few days!

I agree it's all about the experience, more so with plastering, you can't get consistency/mixes right by just trawling forums. Normally I've learnt most of my skills from watching/helping others, can't beat it. I don't know any plasterers though... maybe a trip to Moray/Edinburgh might be on the cards

Thank again, will report back.

 

[vaugi]

Just a quick one to say thanks guys, I've got the wall finished and skimmed. I mixed up by hand on the floor with a shovel, mixed it a bit thicker and also put plasticiser in.

Looking forward to giving it a try again when I get round to the next one from scratch to see if I can manage it in a bit less than 3 days

Merry Christmas!

 

[roughcaster]

Well done Vaugi, you've learnt from that. Keep us updated, and Christmas/New Year greetings to you and yours.

 

[roughcaster]

Duplicated post..