What is the purpose of a base or scratch coat

As I took a walk around the centres at Amaroc I remember having one common thought – why do we have so many basecoats?  Six from K Rend, three from Jub, five from Fassa Bortolo and three from Weber - why?  I took it upon myself to find out from one of the technical members of our team.  Complicated? Well, Yes & no - I narrowed it down to two things:  

  • Understanding the makeup of the render
  • Understanding the nature of the substrate (the wall to be rendered).

    What do I mean by “makeup of a render” - At Amaroc we stock a number of scraped textured renders, all of which perform differently.  The answer is down to where/how they are manufactured.  For example, a cementitious render has a more aggressive drying time after a certain point, where lime based renders have slower consistent drying time. Both types of renders achieve a very similar finish but if the substrate receiving a render finish is not understood it can lead to poor adhesion and hairline cracking.

    By understanding the nature of the substrate and the curing/drying process between render manufacturers, certain brands may offer significant cost savings over others. For example, a render made from natural lime has a more consistent curing process than cementitious renders and as such can be applied directly to certain substrates without the need for a basecoat and/or additional bonding aids.

     The main factors to consider when rendering on your substrate are:

    • Strength – the substrate, including joints, must be stronger than the render and should be able to adequately support and restrain it.
    • Suction – good adhesion of render is dependent on good porosity of the substrate. If the suction is too high the render will not hydrate sufficiently. If it is too little there will not be a good bond.
    • High porosity – this is particularly the case with low-density blocks. The substrate should be lightly sprayed with a fine mist of clean water even before the render is applied. Saturation should be avoided as this will cause excessive shrinkage.
    • Low porosity – this is particularly the case with concrete or brickwork. A splatter or tight coat should be used to provide a controlled level of porosity and a mechanical key.

      For further information contact the Amaroc team who are trained to advise the correct base coat for the best finish. We will ask four important questions

      1. What is the substrate you are rendering over?
      2. What finish does your customer want to achieve? For example, scratch renders, thin coat render, dashing.
      3. Is the substrate inside or out? Area’s more exposed to adverse weather conditions are advised to use either silicone-based thin coats or lime based products with a protective sealant
      4. How many square metres do you need to render?

        With this key information, Amaroc can provide you with a competitive quote and manufacturer specification, to ensure you have another happy client. Contact one of our centres at Bristol, Cornwall and Swindon today!

        April 30, 2017 — Phillip Llewellyn