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rendering interior

Postby ralph brown {jim} » Tue Sep 04, 2007 9:53 pm

hi, only joined today so be patient with me please, the plaster above our 7foot window was really unevan, we stripped it back to the lintel it was 4inches thick at the reveal. we are putting in sliding doors called in the building inspector , he advised us to render the inside face, my question is {after all that} what sand would be best for the job? would it be sharp 2 builders 1 cement 1 ? advice please
ralph brown {jim}
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Simply Build It

Postby Mitch » Mon Oct 01, 2007 8:40 pm


I suspect that there is a misunderstanding here due to jargon, most folk know that rendering is the stuff that the outside walls of your house might be plastered with.

With the recent (ok in the last 20 years) popularity of using plasterboard to "dry line" interior walls, you will find that most people seem to assume that internal "plastering" means "skimming" plasterboard.

But some people might also prefer to have their interior walls "rendered" with a plaster product such as bonding, base coat or hardwall, then finished with a skim coat.

You can of course use a sand/cement mortar to do the job but it would be hard work compared with using a plaster product.

I prefer to use bonding as a base coat, don't forget to use plenty of pva to seal the brickwork first or you will get cracking and fill using multiple coats if a thick layer is required.

Hope this helps.

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Postby rosebery » Mon Oct 01, 2007 9:32 pm

Don't apologise - we all have to be new once. I've only been member for less than a week. Oh and welcome.

Builders doesn't work in rendering - it contains too high a level of clay and the rendering fails after a time. I'd suggest small aggregate sharp sand in place of the usually available (ie all good sheds) stuff. The big bits in "standard" sharp sand can be a real pain when rendering. For the soft sand element use double washed soft sand.

The following seems to work:

Cement - 4, Washed Soft Sand - 10, Sharp Sand - 2 which gives a ratio of 3:1. Use that for the scratch coat and make it 4:1 or even 5:1 for the finish. Use a dash of PVA as well as plasticiser. If you don't want to buy purpose made plasticiser then washing up liquid works just as well - seriously it does and doesn't degrade performance.

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Postby thedoctor » Tue Oct 02, 2007 7:26 am

Although washing up liquid has been used as an air entrainment agent for years we cannot be seen to recommend its use on the site. Fairy and purpose made plasticiser add air to your mix. This makes the mix a lot more workable and so much easier to use. However, no -one has ever been able to measure the amount of air that fairy adds to the mix but it ius a lot more than proper plasticiser. When there is too much air in the mix it is possible for water to get into these tiny air pockets and when this water freezes it expands causing tiny cracks in the surface preperation. This cracks invite more water an so the process goes on. It is called Freeze-Thaw Action and you can read all about it in the projects section. This always incurs the wrath of brickies and plasterers (including us) but it is a fact and as mentioned before, we cannot BE SEEN to be advocating the use of Fairy Liquid in any construction mixes.
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Postby rosebery » Tue Oct 02, 2007 10:03 am

Hi Doc

I didn't say Fairy Liquid and I said a dash not a bucket load. The latter quantity will inevitably degrade performance as you say. Just a smidge makes it just that bit more workable.

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Postby loomo » Fri Nov 02, 2007 6:38 pm

we did a test when i was an apprentice at college and it was found that even minute amounts of washing up liquid in a mortar mix reduce the setting strength of structure's etc by a hell of a lot. .
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Postby canny » Sat Nov 03, 2007 10:22 pm


Use a fine graine sharp sand and lime to a ratio of 1 cement 1 lime and 5 sand.
make sure when you build out the thickness ,the mix stays the same strength ie 1:1:6 or you will get a higher chance of cracking. DO NOT USE washing up liquid, it efects the strength.............only for cowboys!!!
There is an issue using lime with ref to Health and safety,so you can instead use cement plasticiser.
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Postby TheDoctor5 » Thu Jan 22, 2009 10:25 am

Last year 64% of the questions asked in our forum were answered within our DIY project pages at The project pages are now separated alphabetically and your answers are accompanied by diagrams and the ability to see, and buy, the tools and/or required to complete your project. Use our search box to look for your answer and save a great deal of time and money!
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