Basic guide to external rendering

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church

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There are so many different rendering systems these days available to the modern plasterer , and although the material and the way it is applied is changing, probably the most basic system is two coats sand and cement, applied with a steel trowel flattened with a wooden or polyurethane float and finished with a damp sponge. Also covered in outside rendering are different types of finishes using sand and cement as the base material, these are known in the trade as stone dashing, rough casting, to mention just a couple. Smooth rendering although it is one of the most basic rendering finishes also still has many drawbacks which the plasterer has to take into consideration before even starting to render. Bad weather, bad scaffolding, and even bad brick laying will add to the problems you will face when rendering.

Mixes and mix ratios - most sand and cement renders are traditionally mixed in cement mixers, although there is readymix options available for bigger contracts, most domestic and small building firms will still mix the render the traditional way of putting it through a mixer. Different mix strengths are determined through the substrate that it is being applied to, the most basic mix is probably 4 and 1, this is commonly known as a scratch coat, the 4 part being a washed fine sharp sand and the 1 part being ordinary portland cement (OPC). A basic rule when you are learning rendering is that the coats applied get progressively weaker by each layer applied. Also the thicknesses of each layer should be kept within a 6 - 10 mm tolerance, thick coats of render do not necessary mean the render will be stronger, in fact it works the opposite way and normally is an easy mistake to make by the novice.

Add mixes - most first coat (scratch coat) will be mixed using a water profer this product is usually applied directly to the ratio of cement used to the mix, most water profer companies will have the mix ratios printed onto the container, it is important to stick by these ratios. By adding water profer to the mix has a number of benefits to the rendering, one being a controlled even suction for when applying the top coat (butter coat), second coat, this will stop the top coat from drying too quickly, shrinking and eventually causing premature cracking.

Plasticising - again this can be a bad thing if done incorrectly, over plasticising a render mix will weaken it. The benefits of plasticising is to allow the render to be more pliable and easily spreadable. There are liquid plasticisers available but most plasterers will use hydrated lime , this is a more natural way of plasticising the mix again adding benefits to the render, one benefit being lime holds in moisture and allows the second render coat to cure slowly, therefore harder and stronger. Hydrated lime is added directly to the mix and normally gaged using it in the same amount as cement. A typical top coat mix would be 5, 1 and 1, sand, cement and lime respectively. The amount of time the render is spent in the cement mixer mixing it is also important, a common mistake made is allowing it to mix for a long time, thinking it will be better or easier to apply, overmixing the render is an easy mistake to make known in the trade as overplastercising.

As I mentioned at the beginning external rendering is probably the hardest thing for the modern day plasterer to get right, everyone has their own opinion of how to do it and what works for them, the above mentioned is a basic guide to basic rendering.
 
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bigsegs

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nice one church! i think you actually managed to prove that theres no such thing as 'basic' rendering without actually saying it! class ;D
 

ricche

New Member
nelly said:
Lemon fresh fairy up liquid beats the (german word) out of Feb anyday.......its a nice smell while your working.

Theres this rumour that you shouldnt use it and only use Feb...........................said Feb :)
its not just a rumour,but a fact.if you use any washing up liquids it will kill the cement,and can turn the s/c render to dust,in some cases in about 5 years after application.
 

nelly

Private Member
ricche said:
nelly said:
Lemon fresh fairy up liquid beats the (german word) out of Feb anyday.......its a nice smell while your working.

Theres this rumour that you shouldnt use it and only use Feb...........................said Feb :)
its not just a rumour,but a fact.if you use any washing up liquids it will kill the cement,and can turn the s/c render to dust,in some cases in about 5 years after application.
So all them rendering jobs I did 20 years ago useing fairy that are still on the walls ......how exactly is the "dust" still sticking there then?

Feb is an air entraining add mixture! air entraining means it adds air bubbles to the mix, nothing more, nothing less.

Same as fairy does.
 
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