Joining new render to existing render

9 May 2014
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United Kingdom
Hi all

In desperate need of a few tips on this subject ... paranoid that as we come to the end of our long and arduous project, I'll blow it all by cocking up on this very visible last bit of construction.

We've built lots of new extensions and hacked out new areas for windows, and we're left with the job of joining new render to old ... and making it look as seamless as possible. The building is brickwork cavity, with 100% rendered finish.

My builder has suggested rendering up to the old render, then removing as much of the paintwork from the original render as possible, before laying down a sort of mesh used for EWI systems over the whole building - sandwiched between a special type of adhesive (about 5mm deep). You then can paint over this, and he is very sure that with this system, none of the joins will show ...

What do you guys think? Any pro renderers here like to give their take? We simply can't remove the existing render and go from fresh - too time consuming, and also the render is locked solid to the substrate and would destroy much of the outside layer!

Thanks so much - keen to learn ... look fwd to any top tips ....
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Your joins will almost certainly show. But you could disguise them a bit.

Is it possible to use a bead to make a straight line rather than trying to marry in?
Or you could make a raised band over the join to make it a feature.

However, if your chap can go over the lot so that it is all one finish, that is favourite.

This would probably mean removing lose paint, bedding mesh into something like Weber Lac / Krend HPX/Rendaid , flattening a base coat of 3-5 mm then finishing with something like an acrylic 'render' that won't need painting.

Considerationa are cost of going over the whole lot v making the join look like you meant it ie relative areas might determine this.
Here are some pix ... pretty standard stuff I'm guessing.

Must admit I'm confused by my Polish guy saying it's an 'adhesive' that he will use as the final layer (ie. in which the mesh is embedded). Then apparently it is ok to paint over this. The products he likes are from Bolix ... obviously not marketed in the UK under that name I don't think!

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Thanks for the pics,

The detail is fine but there is no context. Could you step back and pic the details from a distance - perhaps shots of a whole elevation(s)?
Your Polish builder is certainly talking Bolix. Get rid of him.
Thanks - but any stepping back and all you'll see is scaffolding!

But to paint a picture with words, the elevations are about 6m x 6m/8m/10m etc., and all contain 1m x 1.5m window apertures, with new bare blockwork surrounding them. A few elevations are 100% new .. so quite easy to cope with those obviously.

Am liking the look of this Weber Lac that Micilin highlighted to me ... will phone one of their reps first thing Monday. Guys must make a start on Tuesday ... that's a must!
I cant see a problem with any good spread bringing that in with new render!!! All that needs to be done is knock off the old render about 9" from the new join and feather it out and put some s/steel ex-metal over the old part and over the new part and scratch and float in as normal.. ;) l dont think l will see what you lads think as l am in a hotel in Liverpool at the moment and flying out to Majorca for a week early tomorrow morning. Still might get on my I-pad if the fishing is no good ;) l would loosen the nut off the bolt take the washer off and chisel out so the washer and nut is below the brickwork then cut the bolt flush with the nut. I hope they are stainless !!
Jeez. I hope they are stainless too Roy! But at £3.50/m I'm beginning to doubt that. Fresh worries. But good point - I will get them bedded back into the brickwork though.

(They are there to fix timber in cavity which door frame is then screwed to ... but this door is under big canopy so X'ed fingers this part of render won't see too much weather...)
Thing progressing ... my guys have notched out (? not sure right terminology!) parts of the existing render so that no long straight joins between new & old. And a lot of the new areas have been rendered too, so that coming on well.

Question regarding paint removal before the 5mm mesh layer goes on top of the whole lot ... can we get away with simply scoring the surface of the existing painted render? The gels etc that are offered for sale would cost me nearly 4 figures to buy! My guess is that removing ALL the paint is a little over the top anyway, and that using an angle grinder to just score/scratch it all up would provide as effective a 'key'.

But am I right guys?
YOu need to get rid of all loose material, whether paint or not, as otherwise all you are doing is sticking to the paint .If it is sound paint and the surface is rough, then tyou will have a physical key.

If you have to provide a physical key, a grinder is good.

Not sure what 'gels' you mean . Something like Weber LAC should not run into thousands! You could also use and SBR:OPC:sand slurry, a search on this site will help you.
Thanks Micilin

It is very sound paint ... but mostly in a sort of smooth masonary finish.

Am I ok to think that grinding it and then priming it will only improve things key wise?

By gel I meant paint stripper ... but turning my back on that now ... no point I feel. But Weber.Lac is actually quite expensive ... £3.80 inc the VAT to do a single m2 ... and we've got 450m2 to do in all!
Thanks Micilin

It is very sound paint ... but mostly in a sort of smooth masonary finish.

Am I ok to think that grinding it and then priming it will only improve things key wise?

By gel I meant paint stripper ... but turning my back on that now ... no point I feel. But Weber.Lac is actually quite expensive ... £3.80 inc the VAT to do a single m2 ... and we've got 450m2 to do in all!

How much per bag is the LAC? Did you cost KRend HPX or SBR?

Grinder to make slots then , it looks like.
Make sure you wash them out to remove dust. Also,try to do it at an angle,
rather than perpendicular.

I would like some kind of slurry or other coat to bond ,even if only SBR:OPC but I am 'belt and braces' , so there are no hollow areas, but I can see no reason why the grinder won't be good enough .
And of course Take your breaks when using a grinder and use PPE.

Anyone else ?
Thanks very much Micilin ...

Price for the LAC around the £20 mark, although I'm waiting on one supplier who is normally very competitive.

I thought SBR made mortar rock hard! We used it between the coping stones at the top of the parapet around the house, but hadn't figured it would be good as part of a 'flexible' mesh layer ...

But KRend HBX looks interesting - although, once again, around the £20/bag area.

Guys have nearly done all the new covering, and it looks good. Going to start mashing up the old paint work on Saturday ... so tomorrow I'm defo going to have to order up this LAC or HBX ... or even Baumit Duocontact might be a contender ... anyone come across that product?!?
Right ... now a new drama:

My nice, crisp newly rendered walls (straight off fresh blockwork) suddenly look (five days after completed) like they've been subject to a massacre. Loads of weeping red/black spots everywhere. Render mix was cement, lime & plasterer's sand (1:1:6).

Now only noticed this today, and preliminary research seems to suggest this is a fairly common west country fault (metallic deposits in the local sand?) ... so does anyone have any top tips on how to stop this in its tracks ahead of us applying the top acrylic coat? If some sort of remedial work isn't done now, I'm convinced the final finish will be compromised, probably quite badly given we are going for a very light finish ...

Wide view

Close up

Really looks shocking!

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