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I have some bricked exterior areas that had some cement render on them, and then paint. The render has fallen off the bricks in patches.

I think the problem is that large chunks of the brick work was done using cement blocks rather than clay bricks as the areas that have come away tend to be more over these bricks than the clay bricks.

The cement bricks though are not the ordinary breeze block type porous heavy versions, but are more aerated and lighter than normal. They also appear to soak up moisture a lot easier (what I suspect is more the problem).

So given the problems with the bricks, what would be the best mix for a render coating that means I do not need to actually replace the bricks themselves?

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I want to help you, but i need to see this, please add some images. –  Asaf Chertkoff Apr 1 '11 at 8:23
    
@Asaf, will try and capture some images on the weekend. –  TravisPUK Apr 5 '11 at 14:44
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up vote 2 down vote accepted

I'd add a mortar plasticiser to a cement to building sand render mix made at a ratio of about 1:4 - I believe your namesake Travis Perkins offer a suitable product ;)

Damp the wall thoroughly before applying the render to slow down the transfer of water from the mortar to the wall.

I guess you could also use a layer of PVA ontop of the wall before rendering to provide some kind of barrier but that's not something I've done personally.

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Thanks for the response bochgoch. I did actually add a plasticiser to the render before applying and the walls were dampened prior. Possibly my mix was not correct. –  TravisPUK Apr 5 '11 at 14:44
    
Oh, that mix has worked for me... what mix did you use? –  bochgoch Apr 5 '11 at 15:02
    
Buggered if I know now, it was some time ago (a few years). I will however be looking to use your mix. What is the ratio of the plasticiser in your 1:4? –  TravisPUK Apr 6 '11 at 11:40
    
did I answer my own question in that last comment? I thought you were referring to the sand and cement ratio as opposed to the plasticiser to sand/cement mix. –  TravisPUK May 13 '11 at 11:30
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Do not use PVA on external walls! Moisture will effect the stability of the PVA and will result in early failure of Render. PVA is only to be used internally as an aid in slowing down the absorption of moisture by substrate from freshly trowelled plaster. –  user13769 Jul 1 '13 at 15:31
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