“Do we really need to two pass with monocouche scraped renders?” | RENDIT


“Do we really need to two pass with monocouche scraped renders?”

Do we really need to two pass the application of monocouche scraped renders and use a serrated edge? This is a question most applicators will furiously debate. The answer to this question I think depends on whether you ever have any of the below issues with any cementitious render product that you apply, whether it be a basecoat or top coat:

1. Sagging, bulges, tearing or slumping down the wall when trying to level off.

2. Splits that appear during initial set before scraping.

3. Holes or splits that appear and need filling during scraping off.

4. Long wait to scrape times and render setting that seems to "hang."

5. Cured renders that sound hollow in small random areas when tapped.

Some applicators who one pass and then spat straight away when applying through coloured scaped renders seem to accept that these issues are part and parcel of rendering and will work around them by either manually and laboriously filling any defects or resort to buying chemicals from third party manufacturers to try to modify the handling characteristics of the material which may have unknown effects on the render itself. There is an easier way to potentially solve the above problems without compromising the performance of render coating and that is by applying your render either over a basecoat, ensuring that a serrated edge is used to remove any trapped air after the application of each coat and before levelling off, or where a monocouche scraped render is to be applied directly to the substrate primed or not by applying in two level passes with the use of a serrated edge straight after the application of each pass and before levelling off. I recommend the 1st pass be approx of through coloured scraped render to be 6mm thick (ISH) with the second pass 10-12mm. Allow the 1st pass to stiffen slightly before applying the second pass. The use of a serrated edge on thinner layers effectively removes most or all of the trapped air and pulls the render together which usually sets it up lovely for levelling off.

I'll further explain my thoughts below:

Firstly, there has been a huge shift in all industries to green and environmental thinking. There’s been massive regulatory pressure applied to all manufacturers to focus on carbon saving on every aspect of their business and this has had a huge impact in product development labs everywhere. Everyone in the building industry is thinking cheaper, lighter, stronger, less energy for everything whether it be bricks, blocks, plasters, mortars and renders. I think for the render industry it’s had a great effect. From a render applicators point of view, today as I type this I honestly think that if you could buy a bag of render from any of the larger manufacturers and you would have a great product to apply. Gone are the days of the huge heavy trailer diesel powered mud-slinging pumps which could only be afforded by the busiest of applicators. These days due to these modern advancements you can buy a tiny but powerful Ritmo spray machine, plug it in and start firing a buttery, light and fluffy dream of a render at a wall 3 floors up in about 30 minutes. Applicators are positively spoilt in this day and age!

The push for green advancements does have a minor downside, especially when we look at the aerated concrete blocks available on today’s market, the push for cheaper, lighter and stronger does mean essentially that there’s more air in them, which in turn means more suction, or variable suction. Too much suction on a render that’s applied to thick on the first pass could result in accelerated drying backwards as the moisture is drawn quickly into the background and there’s potential for random splits occurring in the skin of the render due to the pressure of this pull. Whilst this won’t happen all the time, the risk of it happening is quite high and it is one of the reasons most renders are manufacturer would prefer their material to be applied in two passes. The first pass takes the initial suction hit and quickly pulls in which then protects the second pass from too much background pull. Most manufacturers still suggest a primer or a key coat on very lightweight blocks though as the suction on those things can be huge. Dense blocks need priming also but that’s because there’s not enough suction for the render to create its bond, but that’s for another article!

Secondly, renders in most of the country they have to be finished to a depth of a minimum of 15mm thick to achieve product specification. (20-25mm in some areas.) At this depth it's very easy to trap air deep in the material when one pass finishing as it is being applied with a spray or a trowel. This air may be difficult to remove at this depth, even with a serrated edge so you there may be a risk of air pockets forming underneath the skin of the render which show up as hollows or splits during the set / scrape or the material may sag when it tears at the air pockets during levelling off. You may not even see these problems happening until the scraping process at which at that point of no return they reveal themselves as random horizontal splits, holes, or hollow sounding areas across the wall. Our Mendrend Render Repair technicians can fix this if need be, but it needs a fair bit of work. 

Thirdly, two passing may improve the wait to scrape time, substantially if our site tests are to be believed, as the first pass being thinner will set very quickly which in turn will help the second pass set quicker too instead of one thick insulating pass which may retain moisture for longer.

You should find that the above advice may help with any product that you buy and may be struggling with the above issues but every gang works differently at the end of the day if you have methods that work for you for any given product there's not need to change for the sake of. If you would like more specific advice on the K Rend or Parex products we supply please contact us.

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