Backing coat

Hi Paul
Got your DVD recently and have been practising skimming on a couple of plasterboards. i can't wait to get started properly. Great website aswell. i have an old house(1901) and will probably have to re plaster and skim most walls. I have a few queries about backing coat which I have researched and asked local builders but I am still confused. Here goes -
1. In my daughters bedroom I have had to hack off all the plaster and will have to apply a backing coat to 3 walls. 1 is the inside wall of the outside wall which has a cavity, 1 is a load bearing wall and the other wall is a party wall. I've been told that I should use either hardwall or sand and cement, there is a distinct difference of opinion on this and no-one can tell me why I should use a certain backing coat. What's your opinion on what should be used and why. I'm not that bothered so much about which is easiest, I just want to apply what is best for the house - if I don't put it on right, hell its my house I can hack it off and try again.
2. Most people I have spoken to say s+ c is easiest though some say hardwall is. Someone said if I use sand and cement I should put on a slurry coat - is this a watered down coat, what's the difference, if any, in the ratio of mix between the 2 .
3 . If I apply a slurry coat on is it necessary to use guides
4 . What should be the depths of both backing coats.
5. If I add some sbr , how much per bucket of water.
6 . If I apply hardwall - any special tips for applying it.

Apologises if this is long winded - hopefully it will answer others questions



Hi Steve

It can be difficult to answer such questions as there is no right or wrong answer but hopefully the following may help.

The Main differences between sand / cement and Hardwall are:

Hardwall sets within 2 hours and so immediately presents a problem for a beginner as you are against the clock. Any wall therefore that could take longer than this period of time will have to be carefully thought out.
Hardwall does not perform well in damp situations so if there is any possibility of dampness Hardwall will not help the situation.
But the benefits are:
Easy to mix
Sets to allow finish skim coat to be applied same day.

Sand / Cement is heavier to use and a bit harder to mix but whilst will not help damp problems it will not deteriorate and cause other problems. But the big plus with sand/ cement for beginners is the fact that it has a very slow setting time - giving you hours to work with it. It can firm up very quickly if applied to a very porous wall so therefore a porous wall will need to be damped down with plenty of water or sealed with a weak solution of PVA (this applies to whichever plaster you use including Hardwall)
The other benefit for a beginner using sand/cement is the floating process ( 'rubbing up' or going over with the float). This can be much easier with sand / cement and can rescue a 'poor' wall and transform it to a good backing coat.

To further complicate the issue!
Sand / cement should not be mixed too strong otherwise more cracks are likely (5:1 is a good possibility). On a wall that is built with Lime mortar (as yours may well be, given the age) a good backing coat mix can also contain Lime in the ratio 6 sand:1 Cement: 1 Lime (hydrated non hydraulic lime available from your Builders Merchants) This helps adhesion and flexibility.

The slurry coat is really designed for surfaces that will give poor adhesion, so not needed for a porous standard brick or block wall. Controlling the suction as previously mentioned is normally sufficient.

Depth of backing coat - no thicker than it need be. As long it is thick enough to provide a flat surface to cover the whole wall, this is the main aim.

Hope this helps


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Jun 28, 2013
Backing coat
by: Steve

Thanks Paul for your comprehensive answer.
I'll probably go for the s+l+c option and will let you know how I get on.
If I add lime, how long should I wait until I apply the finish as I understand lime takes a lot longer to set and should I add some SBR to this.

The finish can be applied the next day as the cement will have hardened sufficiently. You don't need the SBR.

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