What to do with this external wall/render?

delmonta

stuart45 said:

You might struggle a bit with the one coat renders, as the aggregate size means they only down to about 5mm.
For a really tight coat you could get some fine silver sand and mix your own with cement.

I don't mind doing multiple coats if it means a better finish. I've used one coat plaster inside, and it didn't finish very well

stuart45

That one looks a bit thin for more than one coat.

FreeBear

stuart45 said: You might struggle a bit with the one coat renders, as the aggregate size means they only down to about 5mm.
For a really tight coat you could get some fine silver sand and mix your own with cement.

I have been using the Tarmac one coat render - The aggregate size is pretty small, and in places, it has been going on at around 3mm thick. If the OP chips off the chalky stuff, going to the recommended 10mm thickness wouldn't be a problem.

Unlike a "mix your own" from sand & cement, a premixed render will contain various additives such as plasticisers, binding/waterproofing agents, and possibly fibres for added crack resistance. As for a DIY primer/bonding, SBA with sand mixed in would be better than cement - The larger grains of sand will give something for the render to grip to.

stuart45

SBR and cement is standard in the trade as a bonding agent. When brushed on the render will have plenty to grip on. Finer sands give a stronger finish with thin coats. Even if you can grind the render to a thinner coat, the finish isn't as strong.
It is not recommended to use SBR as a bonding agent without cement.

delmonta

Thanks. I'm confused now, because it seems that the green you see underneath is just paint on previous render. But only under the bay window is there this layer of maybe 3mm of filler/render...it's light, seems almost like multifinish or something similar.

I'm wondering why they did this in the first place. Why not just paint on the green? This is how the rest of the house has been done

Have a look at the photo to see the layer of filler or whatever it is. 

I'm wondering could I just chip off all this stuff back to the green, and then just pain straight on top of that? I guess there must be a reason they didn't do that in the first place...but the reason could be that they were not thinking straight!