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How to key a wall with a float?

How to key a wall with a float

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Diagram showing how keying helps plaster grip wall Keying a wall involves scratching it all over to prepare the surface to accept the next coat of plaster. If there are three coats of plaster, the second one will need to be keyed.

The process increases the surface area, and means the plaster is pushed into a gap or hole so that it can ‘grab’ more efficiently and lock (like a key).

Devil float with screws in the end Many plasterers use a devil float to key the wall. This is a piece of wood with screws or nails driven into the end.

The screws shouldn’t stick out more than 1 or 2mm from the base, otherwise the scratches will be too deep and could still be visible through the final coat.

Keying wall with devil float Wait for the plaster to firm up sufficiently. You’ll know it’s ready when the surface still feels slightly spongy, but has not yet set hard.

Carefully drag the devil float over the surface in a figure of eight motion. Work your way up and down the length of the wall until it is completely covered. Aim to leave a gap no larger than about 150mm (6″) between scratches, and be careful not to dig the float in too hard.

Using plastic float After keying, wait for about 20 minutes then run a clean plastic float lightly over the wall. This will flatten the surface and get rid of any large burrs that the scratching process might have created.
Scratching brush keying wall You can also use a scratcher, or scarifier, to key the wall. For more information see the Wonkee Donkee guide:  Plasterer’s Scratchers.

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