Question about rendering

[Chapster]

Hi,

I have a question about rendering if anyone has the time to give some advice please.

I lived in Spain for many years and did building work over there, but find everything's different here in the UK. Over there, walls are rendered with a simple mix of 3:1 or 4:1 sand and cement mix, using what I believe is termed sharp sand here, and ordinary grey cement. We used to have to sieve the sand to remove the bits of stone (over there, the sand comes from grinding up white/yellow rocky mountains), to obtain a smoothish mortar.

After a scratch coat, the top coat would be applied thinly and then sponged down or 'scratched' down with a tree saw blade bent into a circle and tied, using the teeth to give a nice rough finish. I don't think the latter fiish would work with sticky soft sand coats...?

Anyway, now I'm over here and I want to render the inside bare bricks in my large outdoor shed (and eventually, the outside). I have no idea what to get and what mix to use. Do I use soft sand or sharp sand, and why. Why does every site I get from Google about this subject refer to using lime, and do I have to use it - and how come it isn't used in Spain for rendering indoors or out? And lastly, what's the plasticiser for and is it necessary regardless of whether you use sharp or soft sand for rendering?

I haven't decided yet whether to have a sponged finish or a rougher one, if that alters which sand to use, or mixture of sands.

Thank you to anyone who helps to clear this up!

 

[Chapster]

Anybody? Pretty please with a blob of cement on top?

 

[roughcaster]

To get best results with cement render, use plastering sand, it will give a neat, close finish to a job whether you use a sponge to finish, or a float. I scratch coat at 4-1-1 sand and cement and lime, and top coat render 5-1-1 sand and cement and lime. Lime is often used in render mortar as it gives flexability to render and reduces the risk of cracking and also gives the mix a good texture and greater workability..I don't know why it's not used in Spain.... You do not have to use lime, it but it helps!! Lastly,, it is "essential" to use plasticiser when using any mortar. If you are rendering, use a quality waterproofer/plasticiser, especially in the scratch coat, to cut down any suction when you apply and work/finish the top coat.

Roughcaster.

 

[Chapster]

Thanks a lot for your reply. Just one thing, when you say to use plasticiser to reduce suction, do you mean to stop the water being sucked out the top coat as it's being applied, making it unworkable? I used to encounter that problem a lot and resorted to what other builders did and soaked the wall with a hose before applying the top coat - which was a messy pain.

 

[roughcaster]

A combined waterproofer/plasticiser will do two things, it will aerate the mortar mix making it lighter, fluffy,easier to work/spread with,(render/roughcast etc) and will also, as you said, control the suction and help hold the moisture in a top coat, making it stay workable for longer. There are other situations where combined w/p would be an advantage, but some where it would not. Both waterproofer and plasticiser can also be bought separately, and used separately, depending on the type of mortar required for the job.

Roughcaster.