Top coat problem

R

RedHerring

I’ve just completed my first rendering job and I had a bit of a problem with the topcoat so I’d appreciate opinions &/or advice.
The wall is concrete block and the scratch coat went on with no problems whatsoever. I used 10mm battens and a darby to help with an even coat. (Something which I learned but have not seen advised anywhere is to run a trowel along the sides of the battens after scratching this first coat. It makes it much easier to release the battens the following day) I used a mix of 1 cement, 1 lime and 4 plastering sand for this scratch coat. Due to my amateurish speed it was between 4 to 6 days before I started the topcoat. No PVA has been used at any time, allthough I was going to prime it before painting with a spray of diluted PVA.
The problem was experienced with the topcoat. I'm talking probably about a total of 1.5 sq metres in 5 patches in a wall of about 20 sq metres. I did soak the scratch coat really well before starting the topcoat and occasionally during. Occasionally the topcoat would not adhere too well to the scratch coat and bits fell off, not on the initial application but later when I was wooden floating it to get a relative smooth finish. I used a 1 cement, 1 lime, 5 plastering sand mix for the topcoat, aiming for about a 5 mm thickness.

Did I polish the scratch coat too much before scratching it? Was the scratch coat mix too strong? Did I use too much sprayed water while floating it with the wooden float? Did I leave it too long before applying the topcoat?
I have to patch the bare bits today. I cleaned the patches yesterday where the bits fell off and wire brushed the scratch coat to make sure there’s a good key.

Another question I have is about the corner angle, stop beads and bell casts,
I used the corner angles, stop beads and bell casts (all stainless) under the scratch coat but it seems to me that the corner angles and stop beads might be better utilised on top of the scratch coat. Whereas the bell cast worked well under the scratch coat. Advice would be appreciated.
 
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Hi Red,

4.1.1 (sand,cement,lime) is fine for the scratch-coat.

Did you use waterprofer/plasticiser in the mix, (important) and was it batched/mixed in a cement mixer?
As long as the scratch coat was evenly and well scratched overall after application, it doesn't matter how smooth it is when you put it on.

You said you soaked the scratch coat before and during, putting on the top coat,,, did you let the water drain off, or did you put the render straight onto the "wet" surface? If you don't let the excess water run off the wall before coating, the top coat can slide/slump. Maybe, when you went over it with your float, these small slumped areas just gave way. You can also get problems if it starts drying in too quick as well,, thats why i asked about the waterproofer. Was it sunny, windy that day? that can affect the job too.

The mix for the topcoat 5.1.1 is ok too.

I never put water onto render when rubbing it up with a float. If you catch it right, there's no need to. I think it ruins the texture and can also make it look patchy.

It doesn't matter really how long you leave a scratch coat, before applying the top coat. As long as it's prepped.

I "don't", and never have, used any type of corner bead (metal/plastic) for rendering/roughcasting etc, i think they spoil the look of corners outside,,,,, my opinion, but i do use stop beads and bellcast. I always incorporate s/beads and bellcast in with the scratch coat. If i "was" going to use corner beads outside, i would do the same with them too,, in with the scratch coat.
You can put corner/stop beads on, then scratch coat, keeping the coat tight onto the bead,, enough to cover the mesh and flat part of the bead, but still leaving plenty of depth overall for the top coat. It's all practice really.

Roughcaster.
 
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R

RedHerring

Thanks for the reply Roughcaster.
I didn't use any waterproofer or plasticiser in either coat. Should it go in scratch or top coat or both?
It was mixed in cement mixer.
When I was soaking the scratch coat, in places it was soaking in straight away, but in other places, where it was well smooth it seemed to run straight off, hence me soaking it during the top coat process. But the excess had run off in all cases before I applied top coat.
Day before yesterday when I was applying top coat was a pretty much mixed day with some sunny intervals and a bit of wind.

Where I've patched it it appears fine now but I think there's many other places that it's going to blow off in the future, usually surrounded by cracks. (Hollow sounding areas) So I'm going to be patching again some time soon.

Well that explains the corner and stop beads well. I used them as a guide to finish the scratch coat level. Next time I'll leave a bit of room for the top coat.

I'm intrigued by the problems that PVA will cause under paint. I've used it before on other's rendering, and I've not been aware of any problems. It seems to make the first coat of painting easier.
 
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Not putting any waterproofer/plasticiser in either coat would have played a major part in the problem Red. The mortar would have been heavy/dead,, no air in it,, even though you used a mixer.
Without w/proofer, the suction from the scratch coat will absorb the water from the top coat render very quickly, even though you soaked the wall.... For next time,,, a measured amount of w/proofer/plasticiser mixed in the water and added to "both" coats, will make the job a lot easier. You would still wet the wall down as before, plus, i would have put the top coat on a bit thicker,, say around 8 to 10mm,, a more workable thickness. Let us know how you get on.

Roughcaster.
 
R

RedHerring

Thanks, Roughcaster.
I've got the other 3 walls to do but may leave them some time yet. At least the most weathered wall is done.
I'll certainly take on your advice next time.
One final question, as it's a garage, the lintel over the door is double blockwork thick and the roller shutter door is fitted on the inside of the inner skin so I have the reveals to render at some time. Now the vertical reveals are gonna be Ok but what about the underneath of the steel lintel? There are some holes in the underside of part of the lintel, the inner half.
Will the render adhere to this part of the lintel and do I fix some expanded metal and/or some angle bead to the outer edge? Or should I leave the lintel unrendered? Perhaps face it with timber?

And I have to say I think this website is brill. I amazed at the time and grateful for the patience of the guys who regularly post.
 
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Render,, (scratch and top coat) would cover a steel lintel if securely cladded with s/s EML, and a bead to finish it off. Make sure everything metal, is treated/rustproof.... Put a few small "thin" packers, (plastic or anything that wont rust/rot),, around 2 to 3 mm thick.. Put them between the eml and the lintel, along it's length, it will allow for a slightly thicker scratch coat to go through the mesh... The job might look good done in timber too,,, a nice contrast.

Roughcaster.
 
R

RedHerring

Thanks again Roughcaster. You guys are an immense help, mentor and motivator.
I'm suspecting I might chicken out and face the underside of the lintel with timber. I suspect I can fix it with minimal number of self-tappers and a suitable adhesive.
 

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