Help with Rendering

dlacheson · 3 Oct 2008

I've recently begun a plastering course at college and although it's early days I've already been given a rendering task. The wall is about 6 feet high and 8 ft long, and is built of concrete blocks.

I know that to prepare the wall I should rake out the joints but I'm unsure whether it should be prepared with PVA.

I'd like to know what the best mix is recomended.

I've seen in other subjects 3:1:1 for the scratch coat and 5:1:1 for the top coat, is this ok?

I would very much appreciate it if anyone can advise me.

kingandy2nd · 3 Oct 2008

If you’re rendering a practice wall in college, I would assume it would be of sound construction and fairly stable (I.e. no blown bricks, etc).

I wouldn’t suggest PVA, just wet down the walls.

If you’re using limecrete, you probably won’t even to wet it down. I assume you’ll have to lay it on, float it off, and then take down again within a short period so won’t want to wait too long before floating.

Don’t be afraid to ask your tutor these things… that’s what he/she is paid for!!

dlacheson · 6 Oct 2008

Thank 4 your reply KingAndy. This isn't a practice wall at college, this is an outside job. I would be using Rendering sand, lime and cement, I only need to know what's the best ratio. Also whether or not to use PVA. The wall is sound an dis about 6 months from being built (I built it!!)

rendereduseful · 29 Jul 2009

Ive been asked to render a barn for sealant purposes...my question is..how long do you leave it between scratch coat and 2nd rendered coat and what is a texture finish??? any advice would help...thanks

The-brickie · 29 Jul 2009

Scratch day one, top coat next day.

A sponge would give a textured finish, I have used an artex roller before for a more textured finish.

rendereduseful · 29 Jul 2009

Hi... thanks for that advice...instead of a sponge finish..I would go with the float finish..unless on texture its easier to paint etc...

The-brickie · 29 Jul 2009

The floats not an easy option unless the render is dead flat.. its why a sponge is used.. You could sponge it, let it set a bit then finish with a float.

Micilin · 29 Jul 2009

Rule it off flat, float it and then sponge it . If you can get a large sponge, like from a cushion or seat, it is better than a small sponge. I use this dry if the render is a bit wet, otherwise a sponge float.

Use SBR instead of PVA outside, if you need sealing or adhesion, but only if you need it. If teh suction is high, you cna hose down as suggested and use waterproofer in the scratch coat

Your mixes are fine, I would use 4:1 for a scratch coat and a 5:1:1 for finish coat

The-brickie · 29 Jul 2009

I personally would never use waterproofer in the scratch coat, end up waiting for hours for the top coat to go off!

Micilin · 29 Jul 2009

On a hot day, or with high suction background, or differing backgrounds?

Try rubbing up an 60m2 gable end in summer with no waterproofer!

You want the render to set instead of merely drying, otherwise it is not curing to full strength

In winter, you won't need it to kill suction, or if you have a small area , I'd agree with you just wet it down. But for a novice, more time with the finish coat the better, wouldn't you agree?

Also , you can tell a client that you have just waterproofed his wall for no extra cost!

Micilin · 29 Jul 2009

rendereduseful said:
Ive been asked to render a barn for sealant purposes...my question is..how long do you leave it between scratch coat and 2nd rendered coat and what is a texture finish??? any advice would help...thanks

To get back to your original question. I would imagine to seal up a barn you would have to use waterproofer in the scratch coat. You should probalby leave 4 days between coats (in theory rather than practice) some specs give 7 days , IIRC.

Are you rendering the inside as well as outside ? Is it brick or stone etc? You should find out about soil levels outside compared to floor inside. You should find out if there is any dampcourse (like old slates, or new silicon injection) or if damp is a problem. Do you need a plinth with bellcast (beading or work to battens. Battens better for sealing up as beads breach the integrity of the render.

Ask if there is any cracking that need meshing, or if movement joints are required (such as beside or over timbers or disimilar backgrounds)

Texture could be rubbed up and sponged (most common), or napped - ask what texture.

You need to google a lot of this, (or look in books!) to gain a bit of knowledge but also to show your tutor that you are interested.

GarethSullivan · 31 Jul 2009

when i was at college all those years ago, if i said to my tutor that i sponge finished a wall, he'd hit the roof! he'd say that sponge finishing is for people who can't render properly!

Don't put PVA on the wall. I rendered an extention where the owner done that cos he thought it would help me. It gave me no end of problems with the render slipping off the wall.

Also you should leave the scratch coat to dry out for as long as poss. as the cement takes around a month to cure properly. if you go over it too soon you risk any cracks that might appear in the scratch coat to come through to the top coat aswell.

Micilin · 3 Aug 2009

GarethSullivan said:
when i was at college all those years ago, if i said to my tutor that i sponge finished a wall, he'd hit the roof! he'd say that sponge finishing is for people who can't render properly!

Don't put PVA on the wall. I rendered an extention where the owner done that cos he thought it would help me. It gave me no end of problems with the render slipping off the wall.

Also you should leave the scratch coat to dry out for as long as poss. as the cement takes around a month to cure properly. if you go over it too soon you risk any cracks that might appear in the scratch coat to come through to the top coat aswell.

Your tutor was right but only IMHO if you were trying to sponge it up without ruling off and floating it.

Nothing wrong with sponging AFTER the wall is rubbed up properly, to giv it that even texture - but it depends what you are after, I suppose.

My old man always says a sponge finish is only for cow houses - a napped finish is the job for anywhere else!

The-brickie · 5 Aug 2009

To sponge or not comes down to two things, time and money.. If somebody was paying me a day rate of 200 a day I'd float to a polish! On the other hand if somebody was taking the pi55 I would get it flat enough as I laid it on and then sponge it, job done and I'm home for tea.

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Help with Rendering

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