Plastering Materials for Backing Coat
There are many different plasters that can be used for the backing coat.
Sand based render mix - backing coat plastering materials
The easiest backing coat for the beginner to apply is the traditional sand & cement render – it stays workable for hours (sets overnight), is very forgiving and always to be recommended. The only drawback is the mixing – which does take a little more effort to mix than the gypsum based plasters, but once mixed this effort pays dividends by giving you all the time you need to work on your wall without it setting. Sand & Lime mixes (cement is substituted with lime) – used in restoration work to give greater flexibility whilst allowing the walls to breathe. More Info on lime.
The ratio of 5 equal measures of sand to 1 measure of cement is adequate for most internal plastering jobs. Water should be added until the required consistency is achieved which is a matter of personal preference.
It needs to be firm enough to ensure that it can ‘peak’ (Make a peak with trowel and it should stay there) If the peak disappears then it is too wet. The peak should be able to be made very easily. If force is required then your mix is too dry.
It is essential that the water you are using for your mix contains a plasticizer. This is a liquid that is added to the water to ensure better workability. If this is not used then you can almost guarantee you will have extreme difficulty in applying your render mix. Adequate mixing is essential to ensure the plasticizer has worked throughout the whole batch of render.
If you have the benefit of a mixer then you should leave it mixing for at least 3 minutes. If you are mixing by hand that is an awful lot of mixing with a shovel so it may be better to mix up in buckets with a whisk powered by a drill. It is worth noting that once the plasticizer has taken effect the mix becomes very creamy and wetter so you may find yourself having to add a few dry ingredients to stiffen the mixture up.
Gypsum Based - Hardwall or Browning - backing coat plastering materials
‘Hardwall’ or ‘Browning – once mixed with water will set within 1½- 2 hours, but will start to firm up and can become unworkable before this time. Is a direct replacement for sand & cement and can be used for all brickwork and blockwork walls. Ideal for smaller areas that can be comfortably completed within the setting time, although it is quite ‘sticky’ and can be harder to use than sand / cement due to the floating aspect being more difficult.
Start off with ⅓ bucket of water and add plaster whilst mixing with whisk until you get the required consistency. No plasticizer is required.
Old plaster or dirty water can bring the setting time down to 5 minutes. so make sure any unused plaster in the bag is properly sealed and if not going to be used within a few weeks - place in a secondary plastic bag to keep the moisture out.
Bonding Coat - backing coat plastering materials
Bonding Coat - once mixed with water will set within 1½- 2 hours but will start to firm up and can become unworkable before this time.
This plaster does what it says and sticks very well to a wide variety of low suction backgrounds such as plasterboard or painted surfaces.
Good for patching and repairing lath & plaster ceilings
Same as for ‘Hardwall & Browning’ above.
There are many other specialist plasters available for restoration work the details of which can easily be found on the Internet or use DIY Help Section.
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The author cannot accept liability for the use of any of the materials or methods recommended in this manual or for any consequences arising out of their use. The author cannot guarantee the suitability of any recommendations made in this manual and shall not be under any legal liability of any kind in respect of or arising out of the content of this guide.