Plastering – Wall Preparation Techniques

First get the surface right. Paint, paper or tiles—all must go over a sound surface, free from flaking paint, loose or cracked plaster. Old paper is stripped after water soaking, as we show later.

Paint that is solidly attached need not be removed, though glossy kinds will need roughening with sandpaper or a wire brush, to give a ‘key’ for later paint or paste.

1 Get down to bare plaster if you can,

and once there, smooth it well. An excellent tool for this is the Rawlplug Flexfile, which slips over the hand.

2 Small cracks are filled with a proprietary filler, one that does not shrink on drying. Scrape out any loose powder first and, if possible, undercut the crack edges so that

it is wider under the surface. Then the filling cannot fall out.

Big plastering jobs

Plastering is a very skilled craft and not one you are likely to learn from books or websites even. Without considerable previous experience no one should set out to plaster a wall. If any large area of wall requires complete renewing then it is best either to employ a professional plasterer or to do the job with plaster-board.

Plaster boards, are large sheets of smooth-faced plaster sandwiched between building papers. These sheets can be applied directly to wooden framework or sometimes to the brickwork of the wall itself. The sheets are perfectly flat and may be decorated without further treatment. The edges of the adjoining sheets are sealed, usually by a self-adhesive canvas strip.

Even this work is really for the skilled worker only. Still, there are occasions when we must do a small amount of plaster work, perhaps a few square feet round some damaged corner near a door or at a window opening. This is certainly possible, provided the job is tackled systematically. Plaster is applied in two or more layers. There is a thick base layer of a fairly coarse type of plaster and a finishing coat which is applied only very thinly, perhaps 1/8 in. thick. The first is often brownish in colour, but this last is usually pink. It is much finer and gives a smooth hard surface.

Plastering is done by applying small knobs of the base coat a few inches apart over the whole area to be covered. The tops of these lumps are carefully levelled to be in. below the desired finished surface. This is checked by stretching a straight edge across the undamaged plaster at either side of the patch. Then the space between the knobs is filled flush with the base coat. Finally the finishing coat is applied, allowed to dry and then re-wetted and rubbed over with a smooth steel float trowel. This gives the final silky finish and enables you to remove small irregularities.

There is no doubt that most amateur work is detectable because the surface is not flush. Making a perfectly flat finish is a skill which only comes with experience, and you may need to make several attempts to get even an acceptable result at first.

For small jobs, you may not need such detailed accuracy. Here, by using a strongly adhesive plaster base, such as Carlite Bonding Coat, followed by a fine plaster finish, a serviceable job can be done fairly easily.

1 Professionals use buckets and boards for plaster mixing. A metal tray like this is easier in the home, preventing unwanted drips and splashes.

2 Smooth the bonding coat on to the wall. If this is very dry, wet it well a few minutes before starting. Keep the coat surface a little below the final level.

3 Slash the soft base coat with the trowel tip to give a ‘key’ for later coats.

4 Finally use a rectangular ‘float’ to smooth on a thin coat of finishing plaster. For best effect let this dry for an hour or so, then smooth it off with a wet float, rubbed in a circular motion.

What if the wall is damp

No covering can cure a damp wall. Impervious sheets, any sort of damp-proof painting or the Protexion aluminium foil which we now describe may hide the damage but they do not cure it. However, where the problem is not so severe as to warrant the cost of a major operation, this concealment may still be useful.

For example, you can paint the whole of the wall surface with various liquid preparations which will prevent damp coming to the surface. Or you can, as we show below, apply impervious films, in this case of metal, which will again prevent the damp from injuring decorations applied over them. We feel that the aluminium foil method is best for amateur use, although some damp proofing liquids also give fairly good results.

Protexion foil application starts by painting the damp wall with a special glue. This is allowed to dry and then a second coat is applied. Before this is itself dry, rolls of aluminium foil are pressed against it. Where the rolls overlap extra adhesive is applied. This aluminium foiled surface can then be covered with a lining paper and decorated with paint, emulsion paint or paper in the usual way. There is no technical difficulty about this job because it does not matter much if the strips of aluminium foil are not quite accurately placed, provided always that they overlap each other. Any tears or bad joints are themselves covered with another strip of the foil.

A most important point is to take care with the inevitable splashes of the black adhesive. This material is specially compounded to kill moulds and to give a very good grip to the aluminium foil. It dries very rapidly and it is practically impossible to remove it from painted surfaces without causing damage. Indeed, if it is allowed to remain on a painted surface for more than a moment or two you may find it impossible to get it off at all For this reason this job should always be done by two people, one applying the paste and the foil and the other keeping the area clear of paste drips.

Using metal foil to conceal damp and preserve decoration

1 Before starting, cover all furniture and the floor with old newspapers. The black adhesive is extremely difficult to remove, especially from vinyl flooring which we have found will become discoloured after only a few seconds contact. Then paint the adhesive evenly over the stripped wall. The liquid is very light so the job is easy, except for avoiding splashes! Keep the tin well stirred as you go along or the black body of the glue will settle out at the bottom. Allow this first coating to dry, which only takes a few minutes.

2 Apply further coat a little wider than the rolls of aluminium foil. There is no need to cut these off to length as one does in wallpapering. Instead start from the bottom and unroll the foil upwards, pressing it gently to the glued surface.

3 As you rise up the wall with the foil you can apply more glue beneath it.

4 There is no need to eliminate all the fine creases which are inevitable in this job but the edges of the adjacent strips must be overlapped by at least two inches, adhesive being spread between the two pieces of foil.

5 Because the foil is so thin, tears are not uncommon and if these occur they must be thoroughly patched. Spread adhesive well over the hole and for two or three inches either side.

6 And then apply a dry patch of aluminium foil over the glue. It is vital that any adhesive which squeezes out of joints on to the surface of the foil be immediately removed with a sponge soaked in a strong detergent solution. Otherwise it will certainly show through decorations which are applied over it. Splashes on wood can be sealed with ‘knotting’ or metallic paints.

The final covering: paint, paper or tiles?

On a sound wall, you can fix pretty well any sort of decoration. But if you are not certain that the plasterwork is firm do not fix tiles or heavy papers. On rough walls,

avoid paint or thin papers. But whatever you decide on, take care with the preparatory work and the finish will just about take care of itself !

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