Guide to Making Good Walls and Ceilings Ready for Decorating

If you’re making a lot of alterations to your house, you’ll probably pull out cupboards, partitions, remove fixings and strip off old wallcoverings. Don’t worry if you cause some damage, since you can quite easily put it right.

No matter what sort of decorating you intend to do, the surface you’re covering must be sound. If you paint, paper or tile over cracks or loose plaster you’re wasting your time. The professionals call this preparation ‘making good’ — and the reason is obvious. Without time spent here the end result will be less attractive, won’t last very long, and you won’t be getting value for the money you’ve spent on decorating materials. Making good takes time, but it is never wasted. Here is a guide for the sort of problems you’ll face in making good walls and ceilings ready for decorating:

Cracks

There are two types of cracks in walls to watch for. A structural one will be large, deep, and often wider at one end than the other — this has been caused by subsidence and you should seek the advice of a professional before any attempt is made to repair it. The second type is usually just a crack in the surface covering of the wall — the plaster, for instance — and because it’s only superficial it can be easily repaired.

For such superficial cracks in plaster, first detach all loose material with the edge of a stripping knife and brush out thoroughly. If more plaster than you bargained for comes away, the plaster must have been weak — in which case, treat as large holes. Fill hairline and small cracks with cellulose filler, bought as powder and mixed with water to a thick creamy paste (mix only small quantities to avoid waste). Smooth it on with a filling knife and sand it down when dry.

In wood, cracks and opened grain can again be filled with cellulose filler — but it will show up rather than blend. If the wood is going to be painted, this probably won’t matter. But if you’re going to finish the wood with a clear varnish, plastic wood or stopping should be used to ensure the best possible finish.

Gouges

These are superficial marks caused by a badly-used shavehook or stripping knife (held at the wrong angle or because it slipped) or an electric abrasive tool which during the smoothing created ridges in the surrounding plaster. Fill as in cracks with cellulose filler, making it slightly proud of the surface. Leave it to dry hard and then sand flush with medium glasspaper. Gouges in wood should be treated in exactly the same way as cracks.

Holes

Small holes are often left when old screws and nails are removed or if wires have been chased in — for these the remedy is simple. Large holes, however caused, can require a lot more attention especially if they’re deep as well as wide (eg, if a partition has been removed leaving gaps in walls or ceilings).

It can boil down to a question of cost — cellulose filler bought in the quantity required for a large hole will be more expensive than a small bag of plaster. Plaster, however, has its own problems —it’s difficult to mix properly, sets very fast and takes some skill to get it to stick to the wall in the first place. In small areas there are ways around this (See Ready Reference on plasters and wall fillers).

Very large holes need to be treated in the same way as plastering a wall — you start with an ‘undercoat’ plaster (it’s much coarser than a ‘finishing’ plaster) to fill to about 6mm (1/4in) from the surface, and this provides a key for two coats of finishing plaster which is applied with a float. As it dries it has to be ‘polished’ by applying water and smoothing with the float. Because of the speed at which plaster dries, this can be a difficult skill to master and tell-tale ridges may remain where the plaster has dried before the polishing began. Experience will overcome this problem.

Large or small holes in a plastered wall first have to be thoroughly cleaned out. Chip out all loose material and undercut the edges with a knife, then brush out thoroughly to remove all the dust.

If the wall is block or brick underneath, and the hole is no more than 100mm (4in) in diameter, then use a small trowel and build up the surface with thin layers of filler.

With a wall constructed of laths (thin strips of wood) and plaster, you first have to expose the laths, removing all loose plaster in the same way as above. But you won’t be able to undercut the edges so easily, so you have to make sure that the filling goes between the slats. If the slats are damaged then treat as plasterboard. Otherwise, build up the filler in layers.

Always overfill a large hole, and to get it flush use a batten (long enough to bridge it) in a sawing action to reduce excess or redistribute it till the required level is reached. Finally smooth the finished surface with a filling knife or trowel, and sand down when dry with glasspaper or an orbital sander.

If holes aren’t too large but are deep, an alternative method is to press in balls of wet newspaper, then skim a layer of plaster or cellulose filler over the top.

If there’s a hole or holes where walls meet to make an external corner, nail a batten vertically along the edge of one wall and fill the hole on the other as described above. When this patch is dry remove the batten and repeat on the other wall. If the damage to an exposed corner is extensive, or if it is particularly vulnerable, greater reinforcement may be desirable. Cut back the plaster as described under weak plaster below to beyond the limit of the damage and square off to neaten edges. Then fix an expanded metal corner-piece to the under- lying wall with dabs of plaster and plaster over it using the batten technique. Internal corners are a bit trickier. There are two methods. Either fill one side, smooth with batten, then leave to dry before doing the other. Or fill both and when semi dry, smooth down with an angle trowel.

For small holes in plasterboard use cellulose filler. Edges of larger holes should be cleaned up with a handyman’s knife and can be covered with a patch of scrim cloth (available from most builders’ merchants and hardware shops) stuck in place with dabs of plaster. Or you can use an offcut of plasterboard (see the photographs above). When secure, gently plaster over using a creamy mixture of filler or finishing plaster and allow to dry. Finally sand smooth.

Large holes in plasterboard must be patched with plasterboard offcuts. To nail in position it will be necessary to cut a hole big enough to expose the nearest wooden supports (in a wall these are called ‘studs’, in a ceiling ‘joists’). On a ceiling, if you can get at it from above, the hole can be cut square and battened along each side, the battens being nailed to the joists. Use 30mm or 40mm galvanised nails to fix the plasterboard in place; then fill in gaps as above.

Holes in wood are best filled with wood, and if the hole is circular, use a piece of dowelling glued in place with PVA adhesive. With some holes, you can achieve the same result with a wedge — hammered into place, and then planed off for a flush finish. Alternatively, use plastic wood or stopping and sand the finish down when dry. If knots are loose and very dry, they should be cut out and the hole filled with a small piece of dowelling, glued in place.

Gaps

Where gaps occur between woodwork and walls (eg, near windows, architraves and skirting boards), a flexible sealant will fill them. Bought as ‘cartridges’, they have a nozzle which can be directed straight into the gap. A ‘gun’ attachment gives even more control and is especially useful in awkward places. The sealant can be painted 24 hours later. Cellulose filler can also be used for gaps but take care to get it smooth. If the gaps are particularly deep partly fill them with strips of folded newspaper and apply flexible sealer over the top. If they’re wide, use thin wood to fill and wood filler to finish, then sand down when dry.

Gaps in plaster cornices (the shaped moulding where walls meet ceilings) occur when a framework (eg, an old cupboard) has been pulled away. Clean up the gap and apply liberal quantities of cellulose filler. When the filler is ‘stiff’ but not hard take a profile comb (you can make this yourself from a piece of card cut to the same ‘profile’ or shape as the coving) and run it along from the existing coving onto the filler. When the match is perfect allow the filler to dry and then gently smooth with the folded edge of a sheet of glasspaper.

Weak plaster

Old plaster may be loose against its backing and will move when you press it. If this is the case in any more than small areas, then complete replacement may be necessary. Unsound plaster will sound hollow when you tap it gently with your knuckles.

The extent of the weak area should be found by tapping, then lines drawn around it with a pencil. Using a club hammer and a bolster, gently chip out the weak area starting at the outside edges of the patch and working inwards (cover the floor below to catch the mess). If you don’t start at the edges of the weak areas and work inwards, you may end up removing half the wall. When the patch has been removed you should fill as in holes. With larger areas you may need a professional plasterer. If the weakness was caused by damp the underlying wall should also be treated with a suitable damp sealant before repairing.

Mould

This may be found in steamy conditions which encourage its growth or where condensation is a problem (eg, in kitchens and bathrooms). Mould appears as grey, green or black spots or patches, and first should be treated with a fungicidal solution. Alternatively you can use a three parts water to one part household bleach solution. The wall should be dry before redecoration. If the problem persists, then you’ll have to tackle the underlying cause — which may be damp penetrating the wall from outside or from below, or lack of insulation and ventilation which causes persistent condensation.

Old adhesives

Where ceramic tiles have been removed tile cement may remain fixed to the wall. In some situations — if you’re retiling, for instance — this won’t matter because as long as the surface is fairly flat any new adhesive will stick perfectly well. In the case of polystyrene tiles on a ceiling there may be dabs of adhesive left when you remove them and the surface has to be cleaned off. The only answer to this is an arduous, bit by bit, removal of each dab. (In places, plaster may come away with the tile or adhesive in which case treat as holes in plaster.) Gently ease the adhesive or cement away from the surface using a stripping knife and a mallet. Then sand the area smooth before decorating. If cork tiles have been taken down, any adhesive remaining will have to be sanded off with an orbital sander — another time-consuming but essential job — if you’re decorating with paint or wallpaper.

Paint problems

If the paint on plaster, plasterboard or wood has flaked, blistered or bubbled scrape off the damaged area with a scraper or a coarse abrasive until a sound paint edge is reached. Wash down the exposed surface, allow to dry and prime before repainting.

If paint on wood repeatedly blisters or discolours, this could mean that there’s a knot there that’s giving out resin. Use a blowtorch to burn off the discoloured part then play the flame gently on it to draw the resin out. Scrape this off, sand the surface and wipe off all traces of dust, then apply two coats of ‘knotting’ sealer (available from most hardware shops) to the patch. It must be dry before painting.

Crazing is another common problem, visible as very fine hair-like cracks in a painted surface. On a plastered wall it’s often caused by applying the paint before the plaster is completely dry. On wood it may be because the paint underneath the top coat had not completely dried. The remedy is to scrape off the surface and repaint.

08. August 2011 by admin
Categories: Plastering, Preparation, Preparation, Preparation, Walls | Tags: , , | Comments Off on Guide to Making Good Walls and Ceilings Ready for Decorating

Follow

Get every new post delivered to your Inbox

Join other followers: