In reply to Trowel Monkey my advice is given on the back of 28years experience in the trade and during that time almost all my work has involved working with sand & cement both internally and externally.
Mixes:
mixes should not be stronger than the substrate you are working on, although this is difficult to keep to when working over blocks as soft as Durox. When working on Thermalite/Celcon blocks I stick to a mix of about 7:1 building sand & cement with Febmix added as a plasticiser.
Thickness:
You can apply upto 16mm in one coat, and as long as you know how to mix a decent bit of muck you won't get slumping when working on Thermalite or similar blocks. Generally you should aim to apply an 11mm coat of render for internal work on flat walls where there aren't any frames to work to.
When to set:
sand & cement shrinks as it cures and if it is set before the shrikage has taken place this can cause the finish coat to crack or be delaminated from the render (even though it may look fine). I leave my internal render for a minimum of 5 days before setting but have left render for as long as 3 months before setting, and it didn't need PVA or wetting.
On one site I have completed 400 houses over a 12 year period using these methods and have had no problems during this time, most people do a job and move onto the next one never knowing what problems have occured a few years down the line such as people stripping wall paper only to have the plaster come of the render as well. When this happens people think that the plaster has only just failed for some reason, but in truth the plaster hasn't been adhered to the render since a couple of days after it was done (as the render shrank). If you want to know a little bit more about my work, my firm is based in Sandon and is a plastering ltd. company and I have a website. Sorry for the length of this post but I was asked.
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