Calling Roughcaster

S

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Hi Roughcaster

Sorry to bother you with this one.

Had all the wet harling chipped off the house - came off clean pretty much everywhere - there was no apparaent bond between the scratch coat and the standard dense concrete blocks.

New roughcaster is going to PVA the walls first to seal the blocks so as to control how much water they absorb when they are re-roughcasted.

He feels that the absorbency of the blocks might have contributed to the failure of the wet harling.

So he will PVA all the blocks and allow to dry. Next day wet walls and apply cement / sand scratch coat.

Is PVA the right product to seal the concrete blocks ? I do not think that he plans to apply a stipple coat using PVA cement and sand. just roller the diluted PVA on.

Look forward to hearing from you.

Many thanks in advance.
 
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Either way,, you need a good key, especially when it render/roughcast,, a lot of weight hanging there, but NORMAL PVA USED EXTERNALLY IS OF NO USE AT ALL. You can get an external grade "Waterproof PVA", (google it) which you can mix with cement to make a slurry key, which can be put onto the wall,,,, left to dry/harden,, and then scratch coat onto that. It's a home made version of SBR.

Roughcaster.
 
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ah i seeeeeeeeee!

sorry RC, didn't mean to sound like i was questioning your far superior knowledge ;)

good info, added it to my memory bank
 
Hi Roughcaster

I followed are earlier posting of yours and sourced Unibond PVA - Exterior Grade - waterproof version. Screwfix stock it.

I am keen to have the stipple coating, but the new harler seems a little reluctant to apply it ?

What will happen by applying diluated PVA to the walls ?

Cheers
 
It wont do any good at all,,, ordinary pva in external situations will come back to life again, and be unstable in damp/wet conditions. You should argue the point about the slurry/key, because it's "that" what'll be holding the harling onto the smooth/dense blocks,,, but use the right product for the job. Others on here, i'm sure, would agree.

Roughcaster.
 
Hi Roughcaster

The PVA that I sourced for the roughcaster is Exterior Grade - Waterproof PVA from Unibond.

I take it that once this dries it can not come back to life when wetted ?

I have uploaded a picture of one section of the wall which was stripped back. You can see the blocks are quite clean.

Would you consider this smooth and thus in need of a stipple coat ?

Cheers
Untitled

Untitled

 
For what it's worth, I agree with the RC. Don't use the ordinary PVA, use the stuff recommended.

(I usually use SBR myself)
 
This might sound a bit contradictory,, but Waterproof PVA is only waterproof when mixed with cement,, it says that on the tin,,, although it's far superior the normal internal grade. As Micilin said,, and what i said earlier on,,, SBR is the ultimate bonding agent in your situation. The blocks themselves do look smooth, with very little key, even on the joints. I'd go for a slurry coat , but it's up to you.

Roughcaster.
 
pva is water soluble,that's enough reason why it shouldn't be used outside.
even on site work ,on internal rendering ,any concrete lintels or columbs had to be stippled with sbr sand mix.
 
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