DIY Plastering – How To Re-Plaster A Wall

Given that it’s a difficult job to do well, plastering or re-plastering an entire wall is not something to approach lightly.

First you must be sure that it really is necessary. And then you should check that there isn’t an easier alternative — drylining for instance.

Why re-plaster?

Patching a plastered wall is a great deal easier than re-plastering it from scratch but often the job that needs to be done needs more extensive work. Because if the patches are large or extensive it simply isn’t worth doing — you’ll never get a surface good enough to paint or wallpaper on.

Large scale DIY jobs present the same problem: if you have hacked the plaster off half a wall it will probably be better to replaster the whole of it rather than simply making good the damaged area.

DIY Plastering, How To Re-Plaster A Wall Walls which are intact but have a poor surface because of previous patching could be another candidate. But in this case you need to assess whether it’s worth the trouble. Remember too that ready mixed skimming plaster is available for resurfacing sound but uneven plaster. This has good adhesive qualities, so you can apply it to painted plaster — something you can’t easily do with ordinary finish plaster.

One of the most common reasons for re-plastering is damp. Plaster which has been badly soaked either disintegrates or ‘blows’ (comes away from the wall). You can check this by tapping the plaster with a piece of wood; a hollow sound indicates damage.

There are several cures for damp walls, depending on what the cause is, and these can normally be easily learnt. Most of them call for plastering skills — the subject of this section. But it cannot be stressed too much that there is no point in re-plastering until you are certain that the damp has been eliminated and the wall given a chance to dry out completely for at least several weeks.

The other all too common reason for re-plastering is that the existing plaster has failed — a problem that can manifest itself in several ways. Cracking is found in all plaster — especially new plaster — because the material has only limited elasticity. But if the cracks become really severe and the plaster blows in some places, it has been incorrectly applied and should be renewed.

Plaster can also fail if it is the wrong type for the wall. It may simply blow, or it could suck all the moisture out of the wall and produce efflorescence — white salty deposits — on the surface. Again, re-plastering is the only cure.

Alternatives to plastering

Faced with the need to re-plaster, you have two choices of how to go about the job: conventional plastering (covered here) or drylining which is a very different method. Drylining consists of covering the wall with sheets of plasterboard — usually fixed to a framework of battens. This guarantees you a smooth surface, but it will only be level over the whole wall if the framework is level — and achieving this is no easy task.

Drylining becomes progressively more difficult the more corners, recesses and door and window frames you have to deal with. But there are also times when it is the only choice, for example if you want to plaster a painted brick cellar wall (plaster does not adhere readily to paint). Usually, the decision boils down to whether you feel happier working with wood and boards or a trowel and hawk.

REMOVING THE OLD PLASTER

DIY plastering is a major undertaking, so divide it into two stages: removing the old plaster and re-plastering. It does mean that you’ll have to live temporarily with a lot of mess, but this is infinitely preferable to a job that turns into a disaster because it’s been rushed.

Start by removing as much furniture and fittings from the room as possible. Cover what you can’t take out with dust sheets, and spread more dust sheets along the route to the nearest exit.

Next work out where to dump the debris. The plaster rubble from an average size wall takes up plenty of space: if you haven’t got the use of a skip or dump, get some strong plastic bags — such as fertilizer or ballast bags — in which to store the rubble until it can be collected.

The wall itself must be as clear of fixtures and fittings as possible. Have radiators unhooked and folded down (or better, capped and removed) by a plumber. Where possible 1 he same should be done for any other pipework likely to obstruct you later.

To deal with power points and sockets, first turn off the electricity at the fuse board or consumer unit main switch. Then remove the faceplates to reveal the backing boxes. These can be left where they are, to be plastered around later. If you suspect that cables are buried in the plaster, make a note of their approximate locations at this stage and remember to go carefully when you start clearing the wall.

Finally, remove all skirtings, picture rails and coving or cornice moulding by prising them away from the wall with a claw hammer or crowbar held against a block of wood. Don’t worry about damaging the wall plaster — you’re about to strip it.

To remove the old plaster, you need a club hammer and a sharp bolster; if the plaster turns out to be a cement-based render, you’ll also need a drill and masonry bit to help loosen it. The ideal dress is a pair of overalls — plaster dust gets everywhere.

To protect your hands wear heavy work gloves and make sure you wear a pair of plastic safety goggles to guard against flying chips.

Start your demolition in the middle of the wall by cutting a V shaped channel in the plaster until you reach solid masonry. (If there are any obvious weak points in the plaster you can start there instead).

LaIf cutting the channel proves really hard work, the wall is probably rendered. In this case drill a series of closely spaced holes in it to give yourself a start.

The purpose of cutting a channel is to give you enough room to angle your bolster in behind the plaster coats and prise them away from the wall. Do this, working outwards in all directions, by tapping the bolster gently —there’s no need to use great force, even with tough render.

Using this method causes most of the plaster to fall away in large sheets. Continue along and across the wall, packing the rubble as you go, until most of the surface is clear. Then hack off the stubborn patches that are bound to remain in crevices in the masonry.

If you come across any cables or electrical fittings make sure that they are firmly attached to the wall before continuing.

Preparing the wall: Although the fresh plaster will take up most of the unevenness in your brickwork or blockwork, it’s no good for filling large holes. Also, unsound mortar joints could cause the plaster to fail later, so initial preparation of the wall is important.

Rake out any crumbling mortar joints to a depth of 25mm with your bolster then brush away the debris. Dampen them with a little clean water and repoint flush with the surrounding masonry using a mortar mix of one part cement to three of soft sand.

Large holes can be patched with pieces of brick. Brush out and dampen them with water before setting the bricks in place with a 1:3 mortar mix.

Preparing for plastering

Professional plasterers have four important advantages over amateurs: they know what plasters to use on any given wall; they have help — in the form of a plasterer’s mate; they instinctively know when the plaster is mixed to the right consistency; and their experience teaches them how to use a trowel — something which can be taught, but perfected only after a lot of practice.

Against this, the doityourselfer has time on his side. If he takes it and uses it well, it is possible to produce an acceptable finish. But if he rushes or tries to imitate the professionals, the result is likely to be a disaster. The preparations described below may seem laborious, but they are necessary all the same if you want perfect results.

Choosing and buying plaster

Selecting the right plaster for the job is part of the plasterer’s art: it can make all the difference between a good and bad finish.

Repair plaster of the type sold in small bags at hardware and DIY stores is no good for this job: you need to go to a builder’s merchant where you can be sure of getting the right materials — in bulk at a competitive price.

Plasters vary in type, but also from region to region — like sand.

To add to the confusion, the terminology is different too. So although the terms below are in general use, always state exactly what you want the plaster for to avoid misunderstandings.

Plaster is nearly always applied in two coats. The undercoat — or floating coat as it is generally called — makes up the bulk of the job; the usual thickness is about 12mm. On top of this goes the finishing coat, with a thickness of around 2mm. Hence the following classifications:

Browning: This is the standard floating coat plaster for semi porous surfaces such as brick and light aggregate block. It has a good filling capacity and does not suck too much moisture out of the wall.

Bonding: The floating coat plaster to use on less porous surfaces such as concrete and dense concrete block. It has good adhesive qualities, which makes it easier to apply than browning. But if you use it on brickwork which is any way damp — and most external walls are — it will suck the moisture straight out to ruin the surface finish.

Finish plaster: Used for the finish coat, it is very fine and hence has good smoothing qualities. But use it to any great thickness and you’ll be in trouble. Finish varies in colour from brand to brand and area to area.

Quantities: All plaster comes in bags of varying size, but it’s the overall quantities that should concern you. The material is cheap, so always overestimate. As a rough guide, for every 600mm square of wall, allow 3kg of floating coat plaster and 1. 3kg of finish.

Cement render: Don’t let this confuse you. Render is a mixture of cement and sharp sand, not a type of plaster. Its main use is on external walls, but it’s sometimes applied in place of floating coat plaster — usually with waterproofer added —,to protect from damp.

Tools and other materials

First and foremost you need a plasterer’s laying on trowel. Most professionals have at least two: one for the floating coat and one for the finish. Assuming that you won’t be doing a lot of plastering you can get by with one. But make sure it is the best quality you can afford

—the cheap type found in hardware stores simply aren’t any good for proper plastering. Internal and external corner trowels are also a good investment: you may be able to scrape by without them, but the extra trouble this causes isn’t really worth the saving.

The rest of the equipment should cause no problems. You need a spotboard to put the mixed plaster on — a piece of chipboard, plywood or blockboard about a metre square and laid on bricks will do. Make up your own hawk for holding the plaster from two offcuts of timber, as shown below.

For scoring the floating coat plaster prior to finishing you need a scratcher: knock about six oval nails through the end of an offcut of timber and then snip or saw the heads off.

You need two standard size plastic buckets

—one for mixing the plaster and one for carrying water. And a spirit level is essential

—for aligning the setting out battens. Other materials: Requirements here will vary according to the nature and scope of the job. For the setting out battens, buy lengths of 12mm square sawn softwood to match the height of your wall. You need enough to divide the wall into bays of approximately 1. 3m width. To go with the battens, get a 1. 5m length of timber to act as a straightedge

a length of planed 75mm X 25mm is suitable. Sight along the piece you choose to make sure that it is straight. And if you decide to plaster an external corner the traditional way, you will need a 100mm X 25mm board.

The modern solution is to use galvanized steel corner strips which you pin in place and then plaster around.

If there are window reveals or recesses in the wall, a scaffold square will be needed. You can make your own, as shown left.

Finally, buy masonry nails for securing the setting out guides, boards and strips.

If you haven’t already done so, prepare the room as you did when stripping the old plaster. Set up your spotboard reasonably near the site so that you don’t have to travel too far with the hawk.

Start with the setting out battens. Take the first one and nail it to the top of the wall. Then lay your spirit level against the side, adjust it until it is plumb, and nail it at the foot of the wall.

Now lay your level against the face of the batten and check that it is plumb in this plane; if it isn’t, ease it from the wall slightly and pack out the low spots with scraps of card or wood.

Refix the batten and recheck that it is plumb along both the side and the face with the level.

Repeat this procedure for all the setting out battens, spacing them at rough 1. 3m intervals. When they are all up, take the longest straight edged board you can find and lay it across the battens at the top, the middle and the bottom. If any are conspicuously proud or recessed, pack them as described above and recheck with the board. When all the battens are level, you know you’ve got a perfectly true framework against which to apply the floating coat.

The next step is to fix any other guide boards or strips. Cut corner strips to size with a hacksaw and nail them in place where needed, checking that they are plumb with your level as you go. If you use an external corner board, nail it to the adjoining wall so that it is plumb, with exactly 12mm protruding on the side you are plastering.

TIP: Immediately prior to plastering, mix up half a small tin of PVA bonding

adhesive in a bucketful of water and use this to dampen — NOT soak the entire wall. This prevents over rapid drying of the floating coat and aids adhesion.

MIXING PLASTER

If you can, get a helper to do the mixing: it’s hard work and if you have to do it yourself you’re likely to get hot and bothered —which is not conducive to good plastering.

Floating coat and finish plaster are both mixed in the same way — in a bucket — in batches of a third of a bucketful at a time, Put the water in the bucket first. Filling it about a third full.

(Do it the other way round and the plaster will clog and go lumpy). Then add the plaster, a little at a time, stirring quite vigorously as you go.

Continue stirring and adding plaster until the mixture in the bucket reaches the consistency of thick cream — really thick in the case of floating coat plaster: a bit thinner than this for the finish coat.

When the batch is the right consistency. Empty it out onto the spotboard, clean the mixing bucket thoroughly, and start mixing the next. Be warned however: you won’t know how much to mix until you know how much you can handle, so start with small amounts only — the most important thing is to avoid the plaster drying before you get a chance to use it. Always reject a batch which looks as if it is ‘going off’.

APPLYING THE FLOATING COAT

When you are absolutely ready to start, mix up the floating coat plaster and transfer it onto the spotboard.

Then scrape a manageable amount off here onto your hawk with the laying on trowel.

Carry the hawk to the first bay — aim to start in the middle and work outwards. Adopt the following procedure each time you apply plaster to the wall:

Tilt the hawk slightly towards you.

Cut into the load on the hawk with the trowel blade, separating a trowel size amount.

Slide this up the hawk with the trowel blade, so that it rolls onto the trowel blade in the process.

Press the plaster hard against the wall and sweep the trowel upwards in a curving movement. Keep your arm fairly rigid so that you work from the shoulder rather than the wrist. Hold the trowel blade almost parallel to the wall, but with the upper edge slightly tilted towards you so that more plaster is fed onto the wall as you move it.

TIP: As you flatten the plaster, it will smooth out. But if you keep pressing and spreading, cracks will appear. At this point stop: it is time to reload the trowel.

Continue in this way until the bay is filled just proud of the battens. You now have t level it against the battens using your timber straightedge.

Hold the timber hard against the battens, a the foot of the wall.

Run it up the wall shifting the straightedge from side to side in a scissoring motion, so that the high points on the floating coat are cut off. Stop occasionally, to clear the excess off the timber.

Always work upwards to cut off any excess; if you work downwards the plaster is certain to pull and could possibly drop off the wall.

Once you haye levelled the bay, fill any obvious low points with more plaster, clean the trowel, and smooth off any places where the plaster has ‘caught’. If necessary, recheck that the bay is level.

Repeat the entire plastering and levelling procedure on each subsequent bay. At internal corners, use the full length of the trowel blade — or your cornering trowel, if you have one — to cut the new plaster neatly. On external corners scrape the plaster against the corner strip or edging board so that you leave a depression about 2mm deep — this is to allow for the finish coat.

After about one hour, by which time the plaster should be semi dry, take up your scratcher and swirl it across the entire surface to leave score marks similar to those in fig. 2. Take care not to apply too much pressure on the scratcher — start gently and increase it gradually until you get the desired result.

When the scratching is completed. Dig out the setting out battens by prising them away against the floor and the ceiling. Fill the trenches that are left with fresh floating coat plaster and smooth this off level with the surrounding surface. Take care not to disturb the surrounding floating coat plaster more than is necessary.

Leave the floating coat to dry for at least another four hours before attempting to apply the finish plaster.

APPLYING THE FINISH PLASTER

This is applied in the same way — and using the same technique — as the floating coat. But the job is much more difficult, because you don’t have any battens to work against. Hopefully, by this time you’ll have had enough plastering practice to compensate.

Aim to make the finish coat about 2mm thick. If you are plastering a large wall, do it in two or more complete sections — you can sand where they join later — rather than risk the plaster drying before you’ve smoothed it.

You will find that the finish plaster spreads and smoothes more easily than the floating coat. As you apply it, take extra care not to let the edges of the trowel blade dig in and you should get a reasonably flat finish.

Inevitably, though, there will be trowel marks all across the surface. Leave these for the moment, until you have covered the entire section to the correct thickness.

You remove the marks and do the final smoothing with a combination of your trowel and a paintbrush dampened with clean water. The exact method is a matter of personal preference: some people wet the trowel blade then smooth the plaster; others wet the plaster before applying a dry blade.

There are two golden rules about smoothing — or ‘polishing’ as it’s often called. The first is that your trowel must be perfectly clean and flat: the second is that you must dampen the plaster only enough to remove the ridges and marks — any more, and you will stop it from hardening.

You have quite a long time to work on the wall — about half an hour — but don’t push your luck. If there are some spots that you just can’t get flat, leave them for filling or sanding later rather than risk ruining the surface.

Finishing awkward corners

Window reveals, alcoves and recesses can be plastered in the normal way, but it’s very hard to get them level. The answer is to separate them from the main job so that you have an edge to work to. Then, when you apply the floating coat, use a scaffold square to check that the plaster is at right angles to the other surfaces and skim off the excess.

Smooth corners with corner trowels, using the same procedure as for the wall.

28. November 2010 by admin
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