Plastering: Patching Large Areas (solid walls)

Before decorating a room some filling will certainly have to be done, but sometimes it is easier and better to put in a large patch, rather than to fill a lot of individual cracks. There are also hollow patches where the plaster has ‘blown’, that is no longer bonded to the wall.

You can recognize ‘blown’ areas by tapping the wall gently; it will sound hollow. Unless these hollow spots are cut out, they will soon craze and, under extreme circumstances, cause wallpaper covering to split.

To prepare a large area for patching, the whole of the damaged plaster must be removed with a bolster, cold chisel and hammer.

First, cut a line around the extremities of the damage with the bolster, then remove the plaster within this area right back to the brickwork.

Then clean up the edge by cutting back a further 25mm; this edge may then be undercut with the corner of the bolster. Clean the area with a damp brush and apply the backing coat of plaster. To do this, mix up the plaster or sand base on the spot board; when it is ready, scrape a convenient amount on to the hawk; hold the hawk against the working area and, with the laying on trowel held at 35 degrees to the horizontal, scrape the plaster from the hawk on to and up the wall. Work from the bottom to the top of the patch.

If the patch is 10mm deep or less, fill the hole level with the surrounding plaster. If more than 10mm, fill to half the depth and, when almost set, scratch the surface to give it a key. When dry, apply another (floating or straightening) coat up to the surrounding level.

Using the feather edge, rule off the plaster to the surrounding level, working from the bottom to the top and operating the board with a side-to-side scraping motion. If there are any hollows, fill them and rule off once more.

Where hemi-hydrate plaster is used, cut back the edges of the patch to about 3mm below the surface before it sets; this can be done with the corner of the trowel.

If a sand-based floating coat has been applied, then the-whole of the new surface must be cut back to a depth of 2mm to 4mm. This is to allow for the finishing coat. This job is best done with a short rule, narrower than the patch, and it is for this reason that the initial cuttingout must be done cleanly. Having cut back, go over the surface lightly with the scratcher, then smooth off the burrs with the wooden hand float.

Mix up the finishing coat and pour it on to the spot boai-d; clean out the bucket ready for further use. Put some finishing plaster on to the hawk and, with the wooden float, skim the plaster over the surface of the work, keeping the strokes as vertical and as even as possible. When the surface is covered, repeat the procedure, this time with horizontal movements, pressing firmly.

Check this coat with the feather edge and, if too much plaster has been applied, rule it off to the required level. Wash the float and work over the plaster in a continuous circular motion, consolidating the plaster, particularly any seams.

Now, using the steel trowel with a little plaster slurry, work over the surface with a vertical pressing motion. During this operation the plaster might tend to drag slightly; if so, apply a little water with the brush to act as a lubricant.

Finally, as the plaster sets, polish it with the steel trowel and wash off the surrounding edges with a sponge.

10. November 2011 by admin
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