My internal render fell of in places

[Chapster]

Hi all,

Sorry to be a post w h o r e, but I'm struggling using materials that, when I first started doing a bit of building and rendering when living in Spain, I'm not used to.

Over there they just have one type of sand which is basically made by grinding down rock from mountains. It's white, very gritty and contains particles from dust size to 1-2mm size grains. Often needs sieving before it can be used for rendering, but when used, it's hard as nails - in fact the sand alone needs smashing up with a pickaxe if it gets wet and allowed to dry.

I found it nice to use and over 10 years of using it on and off meant I got very used to it. It produces a mortar that's a bit gritty but the dusty particles in it make a smooth paste which rises to the surface when floating with a bit of water to get a smooth finish, and it's fairly cohesive and sticky because of these dusty particles. Kind of like an ideal mix between mortar over here and plaster - best of both worlds.

Got 2 bags of sand from Travis, telling them it's for rendering. Seems to be soft sand (it's orange) and hard to render with, it just drags with the trowel and isn't sticky or cohesive. How do you render with this stuff?!

QUESTION IS HERE TO CUT THINGS SHORT:

Anyway, cut this essay short by saying I put my top coat over the scratch coat today and patches fell off while I was flattening it down after it'd stiffened a bit. Weird thing is that the chunks showed grey mortar from top towards the bottom, but 2mm from bottom, where the render was against the scratch coat, it was orange, all cement gone. Presumably this is why it fell away?

I dampened the wall well before doing the top coat and used a 5:1 mix. Is the problem this crappy orange sand that doesn't stick to anything, my rendering skills, or something else?

 

[marshman]

Did you use a plasticizer as It sounds as your mix was water soft and not mixed up right.

 

[Chapster]

I haven't got any proprietary plasticiser so took a tip off this forum and used a couple of squirts of washing up liquid into the mixing water, about a tablespoon per 2 litres. It wasn't a lack of proper mixing as the sand layer was uniformly 2mm of the render where it met the wall, as though the cement had disappeared from just that depth - sucked out by the wall.

 

[dazdel]

Hi chapster,
First, you should use a proper plasticizer, the washing up liquid you have used is way too much, and if over mixed will go powdery on the wall as it goes off, as the sand cannot bond with the cement due to the air bubbles of washing up liquid between the particles of sand and cement, It sounds like you have killed the mix, the other tell tale sign is as you rub it up with the float it comes back wet, and drags. If you must use washing up liquid, only put a tiny sqirt in the mix at the start, that should give you at least two barrows full of muck, depending on what size mixer you are using.

The same happens when you use too much plasticizer as well, hope this helps

 

[external]

suction problem mate,when you damp down a wall,wet it till the wall stops sucking ,till it gives a litte water back.

 

[Micilin]

Also, as well as the lads comments, if you leave it too long in the mixer, you'll kill it- and very quickly if too much squeezy/plas.

You know when it tips in the barrow and see the texture, hard to explain I'm afraid until you see it.

 

[Chapster]

Thanks for all your advice. I think you're all right, too much washing up liquid (it is powdery now) and I didn't dampen the wall enough. Being an internal wall, I couldn't use a hose like I normally would, so I used a spray bottle over a couple of hours in between plastering another wall. Next time it's huge paint brush and bucket of water. Oh, and I will get plasticiser and do it properly.

On a high note (and related to my previous plastering question on this forum), I watched a plastering tutorial on youtube and I saw how wrong my technique was. The proper technique is very physically demanding and got me sweating and knackered quickly! I did the proper long, confident strokes when applying the plaster and made sure I did downward strokes too to fill in gaps, which I never did before., and generally was more confident with the float The result is astonishing - whilst not quite professional, it's the best I've done and I couldn't believe I was doing it. It's actually flat and smooth! Nothing to a pro, but a lot to a DIYer like me.

That's the end of my plastering and rendering for now. Now it's painting and wallpaper time.

But thanks to everyone who took the time to answer my longwinded posts that peppered the forum - you've all helped me do a better job and ensure a better job on future work.

 

[dextrous]

See you soon on the painting and decorating forum

Out of curiousity, any chance ypu could post a link to that youtube tutorial?

 

[roughcaster]

I took a tip off this forum and used a couple of squirts of washing up liquid into the mixing water, about a tablespoon per 2 litres.

The particular so called "tip" that was mentioned, was from a reply where the poster had to patch in a small strip of render around a shower tray that was to be tiled over. He was advised in that post to "use plasticiser in the mix ",, if he had some, but if he didn't have any plasticiser,, for the "very small amount of render he needed to mix up", washing up liquid would be ok.
You would "NEVER" use washing up liquid as a substitute for a plasticiser normally, whether rendering or building.

 

[Micilin]

I took a tip off this forum and used a couple of squirts of washing up liquid into the mixing water, about a tablespoon per 2 litres.

The particular so called "tip" that was mentioned, was from a reply where the poster had to patch in a small strip of render around a shower tray that was to be tiled over. He was advised in that post to "use plasticiser" in the mix "if he had some",, but if he didn't have any,, for the "very small amount of render he needed to mix up", washing up liquid would be ok.
You would "NEVER" use washing up liquid as a substitute for a plasticiser normally, whether rendering or building.

I think as well, RC, that knowing the consistency and properties of the mix you want allow you to tailor things to suit sometimes - like when you get some sand that is very "greedy".

This is where you are better to stick to what it says on the tin if you are not sure, but I dare say there have been millions of bricks laid and thousands of houses rendered with a bit of squeezy - added by people who could guage what they needed (and hidden the bottle under an empty bag) until they had a delivery!

I rarely use plas myself unless the stuff is very dead - it's lime for me where i can , and waterproofer if needed.

 

[joe-90]

The poster's problem is this: Nothing to do with mixing or fairy liquid.
Render 'sets' by chemical reaction. His wall was too dry and it sucked out the moisture before it had time to SET. Render SETS not dries. His DRIED.

 

[Micilin]

The poster's problem is this: Nothing to do with mixing or fairy liquid.
Render 'sets' by chemical reaction. His wall was too dry and it sucked out the moisture before it had time to SET. Render SETS not dries. His DRIED.

Overmixed/entrained render exacerbates the problem, so it could have made it worse.

The render is killed ie does not set properly, it just dries - so the symptoms in the finished job are simliar , and if you have over mixed or too much plas etc in the mix, it makes it worse.

 

[Chapster]

Oh boy, I suppose it was going to happen and now I'm back a few paces and more muck to mix up.

My rendering has blown completely - literally coming away from the wall.

My scratch coat wasn't that scratched due to me forgetting until it had dried a fair bit and doing just some diamond pattern lines with a trowel as it was midnight and kids were asleep in next room. So that probably is part of the trouble, plus everything I wrote originally in the post above.

So I only have to render this wall once more, what should I do to help ensure success? Scratch it more if I can (or scutch it). No fairy liquid this time! Then, should I try to soak like mad and render, or should I PVA at 5:1 or 6:1 to limit suction and then render without further PVA? Can't throw too much water around as it's a bedroom upstairs and I found the water tended to get pulled down whilst in the wall and make the bottom so wet that the muck went really loose and took hours to begin drying.

Or, liquidy cement and sand solution 1:1 painted on or rollered on, and then when dry render onto that after wetting well? PVA, 1:1 sand and cement rollered on then render? I just don't want it to fall off again, I want to get this room done and move on to other jobs!!!!!

 

[Micilin]

So are we back to blockwork or your scratch coat?

Get all loose off . Brush off with wet brush to remove dust etc.

Get some SBR.

Mix a slurry SBR :OPC:SAND 1:1:1

Brush this onto the wall (you could dash this on but if not used to it, use a dustpan brush or some brush you won't need again). Wear gloves and glasses, it's a pain to get off, and make sure you protect (or wash immediately) anything that may get covereed.

Go all over the wall, Covering perhaps 70-80% , leaving it with a rough texture and let this set

You can then (after a day) render on to this.

If your background is even, you will be able to get it right in one coat, but preferable to give it a scratch coat to flatten the wall and fill any humps or hollows, so your final coat is going on nice and even.

Make sure you scratch properly, and do it before the stuff dries out. You want them about 3-5 mm deep and about 10-20mm apart. Don't be so heavy on the scratcher that you are going into the hard background underneath or you will be effectively cutting your render into strips.

You can first coat with a 4 or 5:1 and second coat can be weaker.

 

[Chapster]

Sorry, should have said it's back to the scratch coat.

What's SBR and is it likely to be cheap? On quite a budget unfortunately.