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Decorating Defects - their cause and cure

Cracks in Plasterwork

The art of repairing cracks in plasterwork on walls or ceilings so that the making-good will not subsequently show through the new finish, though by no means difficult, needs a good deal of care and attention to detail. It is not uncommon to find that although the work may look more or less perfect after it has been completed, within a few weeks or months . further cracks will appear, or the repair will show a slight bulge or make itself obvious in some other way.
The trouble is often due to the material employed for the making-good. Many painters rely on plaster of Paris, Keene's cement or Parian for the purpose; while these are not unsuitable, they do not compare with a good brand of specially formulated patching plaster; the latter is invariably of very fine texture while the other products mentioned are of larger particle size.
What often occurs is that the material used first swells as the crystals form when water is added, and then, when the moisture evaporates, shrinks again, leaving the repair visible. In some cases, too, the damping of the surrounding area (which must be carried out to provide a key for the patch ing material) causes the wall or ceiling surface around the cracks to swell, thus forcing the patching medium out into a ridge. This, however, can be avoided by taking care that the affected part is properly undercut.
Defects are sometimes due to the fact that too much patching plaster is made up at a time, so - that some of it crystallises or takes on an initial set before it is used and consequently does not key properly or provide a tight stopping. Another frequent mistake is to try to fill a deep fissure in one operation instead of in three or four; each layer should be given time to set properly before the next is applied and the final layer should be finished very slightly higher than the rest of the surface, to allow for shrinkage, and should be cut down level with sandpaper when dry. Another cause of swelling of the repaired areas is the application of a more or less impervious coating before the moisture in the patch has entirely dried out.
After making-good, the surface should be given a coat of "sharp " white­ lead paint or, alternatively, hung with lining-paper, to even up the porosity before final distempering or painting. If the repair has been efficiently carried out, this treatment should effectively conceal the making-good. In cheap work, where the cost will not permit a coat of paint or lining before distempering, it will often happen that the areas which have been patched will show distinctly through the new distemper. This is due to the fact that the porosity of the patch is greater than that of the rest of the surface.
While it is often difficult to effect a complete cure for this condition, the latter may be improved as follows: if the difference in absorption between the two parts of the surface is very marked, allow the patch to dry and coat it with shellac knotting, or, if Keene's is employed for the repair, with a coat of sharp white-lead paint. Allow these to dry and then distemper the whole of the surface.
If the difference in porosity, though sufficient to cause the areas where repairs have been carried out to show through, is not very great, apply the shellac knotting or sharp white-lead paint as above, but while either of these two coatings is still wet, wipe off with a clean cloth. This will satisfy the suction, but to a less extent. than if either film had been allowed to dry without being partially removed.