Render over render

[MadAlicesDad]

I need to render an outside wall on my extension. Most of it is already rendered, but the top 12" is new brick where I raised the eaves. The render is sound apart from a couple of small patches around a window and where there used to be a pipe through the wall.

What I would like to do (pending advice to the contrary) is fill the holes and to apply a scratch coat to the new brick to bring it all up to the level of the existing render, then apply a single coat (5:1 mix about 6mm thick) over the whole lot.

Is this sensible, and if so, what preparation is required for the old render?
It's currently painted with Sandtex masonry paint which I was proposing to go over with a brush in an angle grinder. Will this provide enough of a key or should i score it with an angle grinder disc as well?

Or should I just bite the bullet and knock it all off and start from scratch?

TIA
Mike

 

[PrenticeBoyofDerry]

It's not a sensible idea to render over render, the bond is likely to fail as it needs to have a good surface to key to and the additional coats will put additional stress to the existing coats.
If it's not worth a complete knock off, it is worth considering base/scratch coating the new brick work and damaged areas, then blending in your top/finish coat, then repaint the walls.

 

[MadAlicesDad]

I started when I got back from work tonight. I've just finished the job, cleaned my tools and come inside.
I hope you're wrong!
Mike

 

[jrplastering]

so and i dnt mean this in a narky way but how comes you asked how to do it if you knew already. i wouldnt think it will stick very well might have had more chance with a sbr splatter coat, what did you do?? i.e mix ratios and how many mixes and such forth

 

[MadAlicesDad]

No problem, it's not narky, but note that I didn't ask how to do it, but whether what I intended to do was inadvisable. From experience, good advice and replies on these folders come thick and fast especially if someone is about to do something wrong. After several days there were no replies saying don't do it and as I needed to get the job done, I got on with it in the way I intended.

For info, I applied a scratch coat to the unrendered bricks and damaged areas to bring up to the level of the render, then went over the render with an angle grinder brush to take all the paint and dirt off. I then scored it all over about 2-3" apart with a diamond disc on the grinder, hosed it down and scrubbed with a brush to get the dust off. I drenched the wall with a hose and applied a single coat of 4:1 mix. So far seems to be stuck alright.

On the positive side, if it falls off, I've only wasted 2-3 hours of my life and about £10!

Cheers,
Mike

 

[jrplastering]

that dnt sound to bad actually mate i reckon you will get away with it, but did you put waterproofer in the scratch coat?, make sure your next coat is a 5:1 mix fella, any more questions you need answering or you ok with the info you got?

 

[MadAlicesDad]

Thanks for the positive view. If it fails, I will admit it and let you know!
Next time, I'll leave more time for the experts to reply - this folder is a mine of useful info, and info from pro's is priceless.
Cheers,
Mike
PS there was waterproofer in it.

 

[jrplastering]

i reckon youll be alright mate, i might have given it a spaltter coat first just to give a nice key for the first coat, but you cut the grooves so youll be ok