Internal backing coats - a basic guide

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Ignoring plasterboard, theres basically 2 types of internal backing coat..
Gypsum based
Sand/cement based

Gypsum based coats these days are usually used in newer houses which tend to be constructed of concrete block or concrete and are split again for differing levels of suction.
For poured concrete or engineering brick Bonding would be used, its designed for low suction, and sticks like nothing on earth. In the case of NO suction, apply wba or thistle bondit as decsribed in 'creating a key'
For concrete block construction hardwall or toughcoat would be used, these are for medium suction backgrounds and are generally harder setting than bonding.
For lightweight blocks such as durox, drylining with plasterboard is usually used as the suction levels are exceptionally high however, if you need to use hardwall then the blocks will need a method of suction control (gypprime, pva or plenty of water)
The method is similar to rendering, only with one coat.
The mix is layed on to a depth of 8-10mm, ruled flat with a darby or preferably a straight edge.
When it starts to firm up a sponge float will consolidate the surface nicely and a devil float should be used (although devil floating tends to rip your backer to bits if your on it too early) leaving a surface that will accept finish plaster. Finish plaster should be applied the same day as soon as the backer changes colour. Leaving the backer more than a few hours will increase the suction levels way beyond that which is suitable for finish and youll find it drinks pva (if you leave it a day) to bring the suction levels back down to normal levels.
Some experienced plasterers just rule it, close it in with a darby and skim it, it works but devil floating is recommended)

Sand and cement backers such as limelite renovating plaster and (cant remember the other one) are more suited for renovation work on older houses with a common brick substrate.
Application is much the same, lay on and rule off however a wooden or polyurethane float should be used to consolidate the surface. The drying process takes longer than gypsum backers and devil floating is essential to create a key. Devil floating also does another less obvious job : as your skimming it you might well pick up little grains of sand in the mix off the backer, these will end up in the grooves created by the devil float and not leave lines in your skim (annoying at best)...
These should be left for 12 hours to dry before you skim, usually day after.
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