Best way to render in preparation for tiles

[Micilin]

To take it to extremes,

would you rather have a house made of sand with concrete stuck to it, or a house made of concrete with sand stuck to it?


I think that makes sense???

 

[Albumen]

sort of.

Though some places seem to say each successive coat should be weaker either by making it a weaker mix, OR by making it a thinner coat.

It was a fairly thin coat I put on. I'll cross my fingers that it was 4 to 1, maybe a bit stronger. And do the next one 4.5 to 1 by weight.

 

[Micilin]

sort of.

Though some places seem to say each successive coat should be weaker either by making it a weaker mix, OR by making it a thinner coat.

It was a fairly thin coat I put on. I'll cross my fingers that it was 4 to 1, maybe a bit stronger. And do the next one 4.5 to 1 by weight.

You want the more rigid coat on the inside, and coats to become progressively weaker as they are put on top of each other.

It would make no sense to affix a more rigid part of a structure to , and want it to be supported by, a less rigid one.

Think of the building wearing layers of clothes - it would be no good having the most stretchy garment on the inside and a non stretch one on the outside.

 

[Albumen]

Yes you're right, a lycra body stocking on top of a royal navy duffle coat would look bloody stupid <g>

But I get what you're saying. Wish I'd been a bit more precise with my first mix now. I suppose doing it by weight from now on would actually be a mistake. I ought to do it by the shovel full, like I did at first, but less cement each time.

 

[Micilin]

Yes you're right, a lycra body stocking on top of a royal navy duffle coat would look bloody stupid <g>

But I get what you're saying. Wish I'd been a bit more precise with my first mix now. I suppose doing it by weight from now on would actually be a mistake. I ought to do it by the shovel full, like I did at first, but less cement each time.

My experience of lycra is fortunately very limited.

 

[Albumen]

Sure, but what an image eh?

One more question though. There's a little part of this job where to bring the wall out square I'll need to put a thin coat of sand and cement on top of finish plaster. Do I need to PVA the finish plaster?

 

[roy c]

Just a little tip for you, when the lads say 4-1, 5-1, 5- 1- 1, these are measured mixes ie: mixed in a container ,be it a bucket or another reciporical, if you want consistent mixes you have to measure them this way!
Using a shovel will not give you a consistent mix.... Also the additives that you use must be gauged by what it says on their containers...

 

[Albumen]

Thanks. I'll do it like that from now on. And just hope that the first mix was strong enough. I think I erred on that strong side, especially considering the sand, even though it was bagged, as wet through.

 

[roy c]

There are times when you can not go on weight of material, as cement will always be one weight and sand will not. But if you do all your mixes by the bucket at least you have a good yard-stick. And if the sand is soaking wet and I mean sodden (in bags) and you can see the water in the bags ,don,t buy them! You can always cover loose sand and bulk bags....I tend to try and keep all the sand I use covered over, as you never know what nasties lurk in our rainfalls....

 

[Albumen]

Yes, will pay more attention next time I'm at the builders yard. I opened the tailgate of the Volvosaurus and they said 'oh, sbr's in the office, we'll load the sand'. Wasn't till I got home that I found it was wet through. But then, does sand hold much water?

 

[roughcaster]

does sand hold much water?

It really does!!! Cover it up as Roy said. When sand is wet, it's harder to measure the render mix accurately.

 

[Albumen]

That's good, in a way. Cause it means the mix I put on the wall was stronger than I thought.

By the way. This SBR stuff, it wasn't still tacky when I put the scratch coat on (though it had only just gone I think). Problem?

 

[roughcaster]

If the sand is too wet Mikey, it's easy to add "too much" to a batch, making it a weaker mix, but looking at your pic, it looks ok. It's always better to coat onto sbr while still wet/tacky. It's looking good though. With your deviling float, on a small area, i'd just use one nail it,, you'll still get a good enough scratched/keyed surface on the render.

 

[Micilin]

does sand hold much water?

It really does!!! Cover it up as Roy said. When sand is wet, it's harder to measure the render mix accurately.

For example, when turning over really wet sand to mix up screed, you sometimes need to add little or no water at all.

 

[Albumen]

You're all giving me a lot of help here, which I do appreciate. Thanks chaps.

You see, in a former life, like 30 years ago, I worked for a jobbing builder, and I'd say a year or so was spent labouring for a plasterer. As you know, 1 plasterer + 1 labourer is generally fairly easy on the labourer, unless the plasterer is really shifting on browning. Or maybe I just had a slow one! But he said, one day, 'get a float and hawk and I'll show you a bit'. So I did and he did. And I've done a bit, but not for 25 years. And TBH couldn't really get a perfect finish with skim. But at the back of my mind was a memory of trying to render once and it being a disaster. Hence my apprehension on this.

But everything went swimmingly yesterday. Very little on the floor and NO CRACKS ANYWHERE. Not around edges joining on to exisiting plaster. Hopefully a combination of SBR, waterproofer in the mix, and plenty of spraying of water. My render hasn't 'shrunk away' from any of the existing. Unlike his work. No shrinkage cracks atall. It went off slowly. I've got one of those garden spray thingies, and once it had firmed up a bit gave it an occasional very fine light spray. No fires in the room, no direct sunlight. So hopefully it's drying slowly. And curing properly. Scratched up nicely.

I've just worked out what my mix was, by weight (I've got the bags and part used bags and old bathroom scales). I reckon 4.3 to 1. That's wetish sand to cement. I'd call the sand damp rather than wet through. It holds it's shape, but is still a bit crumbly. It's not sodden.

So allowing a little bit of weight for water in the sand that's getting me pretty close to 4:1. So all future mixes by weight a bit weaker, yes?

How long should I wait for the next coat?