Type of sand for backing coat -- again ...

by Craig

Hi Paul, I know you have already answered a question on sand grades, but..

I was going to buy 'Building' sand and then noticed that 'Plastering' sand is available at three times the cost.

We have a 100 year old house and have knocked the plaster off back to the bricks, I was going to board it, but that means re-fitting all the lovely old skirting and moving radiators.

Having watched your videos many times, I have now skimmed a bedroom, a living room and a toilet (with progressive success). The new skimmed plaster (over the old) in the toilet which was my first job now has a few hairline cracks in appearing which I want to avoid to the hallway, hence knocking it all off and not patching.

Can I use 5/1 standard building sand/cement for this? Also, would I mix it like a skim coat, adding the dry ingredients to water, or mix the sand and cement together first then add the water (will be in a plastering bucket with drill and whisk not a cement mixer).

Final question, sorry, the wall is 9 feet high, and around 20 feet long, I had a cunning plan of fixing battens to sections of the wall ensuring they have the same protrusion along the wall, which I could then drag the straight edge over ensuring I don't dig in etc and all is level, then removing these at the end and filling and floating the tracks left, could this work or is a daft idea and better to persevere with the method you show in the backing coat video and hope I get it level?.

Many thanks


Hi Craig,

The coarser the sand the better - for example less cracking. Building sand would be okay to use, but what can be more important than the type of sand is the strength of your mix. The backing coat does not want to be substantially stronger then the wall. If your house is 100 years old there's a possibility it's built with lime mortar - which can be slightly more flexible. Applying a strong sand/cement mix may result in cracking, so substituting some of the cement for lime can be beneficial. A 6 sand:1 cement: 1 lime or weaker may be worth considering. Hydrated lime can be used for this which is available at builders merchants.
If you're mixing in a bucket it may be easier to add the dry ingredients to the water as you are whisking.

Probably best to use whichever method you feel comfortable with, the procedure I explain is fairly foolproof and helps to develop skills, however if you are only ever likely to plaster this wall and don't need to develop your skills then the method you describe will be fine - however it could be quite time consuming. But, it's the end result that matters - doesn't matter how you get there!

Good luck

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