“Do we really need to two pass?” was a question I got asked by a customer a couple of weeks ago, the question was then followed by the statement “We’ve never had to before!”
The answer to this question, as I explained to my customer is yes, when you aren’t applying render over a basecoat and for a number of pretty big reasons.
Firstly, the biggest reason is science. There has been a huge paradigm shift in all industries to green and environmental thinking. There’s been massive pressure applied to all manufacturers from above 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 Ritmo, 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 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 specified, including those from K Rend and Parex 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 still suggest a primer such as K Rend R10 or Parex Micro Gobetis 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!
The other main reason for two passes being specified during render application ties somewhat to the reason above. The proliferation of modern light, fluffy and extremely easy to apply renders compared to the heavy unwieldy renders that were around not so long ago has left applicators spoilt in these days of Facebook, Twitter, forums and social media. Applicators who have a bad day applying a product that they can’t get on with are quick to tell their mates or anyone else who reads their social media accounts, which in turn can be shared many thousands of times in applicators circles very quickly. So whilst science is making these products go green, at the same time science has to pretty much create a render product that essentially puts itself on the wall compared to the renders that were around just over a decade ago relatively speaking.
An amazing feat when you think about it which all of the large manufacturers have pretty much nailed in my opinion. There’s been pain getting to where we are today, but today as I type this we are in a very good place with application quality of modern renders!
The only downside to a light and fluffy easy to apply render, is that in most of the country they have to a finished depth of a minimum of 15mm thick (20-25mm in some areas.) to pass water absorption criteria set them by building regulations, but they simply don’t have the structural strength during the initial application from the spray to be applied at that depth in one pass without the risk of sliding down the wall and risking horizontal splits or tears as it sags under its own weight. You may not even see these happening until the scraping process at which at that point of no return they reveal themselves as random horizontal cracking across the wall. Our technicians can fix this if need be, but it needs a fair bit of work.
Best to be avoided if at all possible.
My customer ignored my advice.
We’re on site to sort his problems in a few weeks, he doesn’t get paid by his customer until we’ve done it…………..