Problem with external render

Posh Boy

Hi,

Need some help and advice please. Was doing some external rendering the other day. I had previously put on the scratch coat no issues. I now wanted to put the top coat on.

I mixed the sand and cement (ratio 6:1), added plas and water to the mix and away I went and applied the top coat.

I flattened it off and let it go off. I then went back to float it and when I tried with the float as I rubbed the whole thing just started falling off like sand. I though maybe it was too dry so I misted it and tried again - same result.

Any ideas on what has gone wrong? Is the mix too weak? Honestly, it was like I was sanding the wall the way it all just started coming off as I floated it…….

10-10-20

Was it fresh cement and was the sand graded for plastering? And was it hand mixed or machine?

It happened to me with a patch job 2 years back and I realised that I added in soft sand to the mix and probably should have increased the cement ratio as a result. Sounds like yours was just too light on cement.

Posh Boy

https://www.boards.ie/discussion/comment/122589754#Comment_122589754

Yeah, cement was fresh and it was plastering sand I had ordered so I am assuming that was right, used a machine to mix it.

Anyway, you might be right about the mix being a bit lean, I took it off over the weekend without too much difficulty.

I'll try again with a 5:1 ratio mix and see how it goes.

Just wondering about how wet to make the mix - is there a rough guide on how much water to add or what it should look or feel like? and how long to mix it using the mixer?

Also, a section which I had done but not floated went off a bit too much to be removed easily, I left it as is since I can go over it with another coat, except it's not scratched. Someone said to use a water and ploybond mix on the wall (or in the mix) to get it to stick to the relatively smooth surface - any thoughts?

10-10-20

https://www.boards.ie/discussion/comment/122597710#Comment_122597710

I'm not a plasterer BTW, just a DIYer, but in terms of the water a lot of it depends on whether you use a plasticiser or not. The recommendation is to as it reduces the water requirement and makes it flow and float better due to the additional air-entrapment. As far as I know the guidance is to avoid a waterproofer in the mix though, unless you specifically need waterproofing at that point in the make-up for the wall (ie, double-leaf walls are supposed to be able to dry out, waterproofer prevents that).

Another thought came into my head, and that's if the plaster dried off too quickly - the cement will not have cured and weakened the render. I had to cover mine in a tarp when I did it as it was warm and sunny with a bit of a breeze. I also ended up using a low pressure spray to keep it damp, just enough to dampen the wall and not to pull the cement out of the mix.

The mix needs to be soft enough that it's workable but not readily flowing. You should test it on a float to see, it shouldn't turn to jelly when the float is shaken, but not be too stiff to press into the wall.

10-10-20

Oh and Polybond PVA… don't use that. Look for SBR and use it in a diluted state. It will reduce the suck on the old render, but you will will need a key, so give it some sort of a scratch. I used an angle-grinder to put a key on one part which I messed up.

Posh Boy

Thanks for the advice, every little bit helps.

Was thinking about trying to key the surface myself as opposed to using any kind of bonding agent.

I got more plas too over the weekend so maybe I was a bit light on that too, I'll give it a go and see what happens.

For me, getting the mix right was the hard part initially as its more of an art than a science so a lack of experience is definitely not helpful. I am getting better but occasionally I get it wrong and the mix is just too wet. Live and learn eh …….

10-10-20

Don't overdo the plasticiser too much - only a small amount is needed and an excess does not benefit the mix.

standardg60

https://www.boards.ie/discussion/comment/122598026#Comment_122598026

6:1 sounds way too weak for me, would be more for paving. 4:1 is what I use.

As for ending up too wet, don't fill the mixer all in one go, start with a half load, then add the rest gradually keeping it on the dry side. Last bit of water at the end, you want the mix to run up the side of the mixer before falling at the top, good luck!

Posh Boy

https://www.boards.ie/discussion/comment/122604778#Comment_122604778

and would it be ok to use 4:1 given that the scratch coat is already 4:1. I thought the top coat had to be weaker

standardg60

https://www.boards.ie/discussion/comment/122606608#Comment_122606608

Just not stronger, same ratio is fine in my view, and easier to apply.