Painting exterior wood and metal

Painting the outside of your house is by far the most demanding and time-consuming type of decoration you can do. It is obviously not work you want to repeat often so careful planning, the right tools and materials, and an extremely methodical approach are important. This way, you can be sure of an attractive and durable finish.

When to paint

Although spring is the traditional time to paint the outside it is not necessarily the best time. If the exterior wood and metal need painting in the spring, no doubt they needed it in the autumn; and the ravages of winter can worsen the condition of the paintwork and make your job far harder.

Early autumn is a good time for painting, providing you can be sure of finishing before winter sets in. Should the work be delayed by bad weather, conditions can only get worse and you will probably have to leave it until the following spring. Days are also shorter at this time of the year and evening painting is rarely possible as the light fails.

If you intend to paint in the spring, it is worth a weekend’s work in the autumn scraping back peeling paint from the timber and metalwork. Coat the patches with an oil-based primer to protect the surfaces during the winter months.

Ideal conditions for exterior painting are rare. If you try to paint when it is raining, the water will prevent the paint getting a good grip on the substrate—the bare surface—and after a short period it will flake or peel off. Rain falling on a recently painted surface forms small craters which reduce the gloss and spoils the appearance very rapidly.

Even a heavy dew forming on a paint film before it is dry can cause the gloss to disappear completely. And if varnish is used in these conditions, it may turn milky.

In late summer or autumn, it is best to finish painting about two hours before sunset to reduce the risk of condensation.

Extreme heat causes the paint to set very quickly and reduces its flow as you apply it—resulting in severe brushmarks or a textured, ‘feathered’ finish. High temperatures also thin the paint film, reducing its effectiveness and covering power.

Painting in high winds should be avoided as they carry dust which deposits on the wet paint and spoils the finish. And winds make any ladder work very precarious.

Cold weather makes the paint thick and hard to mix, so runs are more likely to occur. It can also delay the drying time of the paint and you may have to wait up to two days before applying a second coat.

The worst areas

Exterior paintwork rarely deteriorates to the same degree throughout a building. You will probably find that the paintwork on the wall of the house which faces the sun breaks down quicker than on other aspects. It is here that you should concentrate your renovation work—a complete repaint is not always necessary.

The first sign that a paint film is coming to the end of its effective life is when the gloss disappears. The paint becomes porous, and severe winter frost and rain will quickly penetrate it. If you prefer to do the whole house at one time, try to make up for the relative levels of deterioration by putting an extra coat on the most exposed or sun-facing surfaces and on any other face which shows signs of rapid breakdown.

Checking for damage

Check all surfaces very carefully before starting preparations: the paint film which appears firm can often hide a decaying substrate. Usually, flaking paint indicates that the surface is wet, rotting or rusting.

Scrape the paintwork and prod the timber surfaces to ensure that there is good adhesion and sound material underneath. If the timber is rotten, it must be removed and replaced with sound, seasoned wood. Paint does not halt corrosion or wood rot and to ignore such defects will result in very expensive repairs in later years.

Investigate the sources of damp around affected timber and put them right before starting to paint. Damp may be entering through porous brickwork, a broken downpipe or maybe a perforated damp proof course. Replace andy broken windows or damaged putty and check all mastic or caulking seals.

Where to start

Always start work at the top of the house so that at no time are you painting, or preparing, above finished work. Your main objective is to avoid having to move the ladder or tower scaffold more often than is absolutely necessary. Plan your sitings carefully before starting so that you can safely reach as large an area as possible and carry out as many processes as you can in that position.

It is not a good idea to apply two coats of oil-based paint in one day, but you can leave the scaffold in position and apply the second coat the next day. Afterwards, move the scaffold in reverse order back along the house.

Priming or undercoating can usually be carried out as soon as the surface has been prepared, so avoiding a scaffold move between preparation and painting.

Metal gutters and pipes

Cast-iron or mild steel gutters and cast-iron drainpipes rust if they are not properly protected. Always take the opportunity to clean out and paint the inside of gutters when you paint the outside.

Scour the surfaces of both sides of the gutters with a wire brush. If rust is left on the surface it will continue to form under the paint film and eventually cause flaking. Young rust can be detected when there is a dull shine on the metal. This should be removed as well.

When you have cleaned the surfaces, dry them thoroughly with rags or carefully pass a blowlamp over them. The bare metal areas are then ready for immediate priming before any moisture can settle and restart the rusting process. The primer should overlap the sound paint around the derusted areas by about 5mm.

When the primer on the bare area is dry—after 24 to 48 hours—wash all the surfaces with a detergent solution to remove accumulated grime and dry them with a rag or chamois leather. You can then apply an oil-based undercoat to the primed areas.

After the patches of undercoat are dry—at least eight hours later—apply undercoat to the entire surface. When this is dry follow it with a coat of gloss paint. For gutters, 25mm is th’e handiest size of brush.

Although you can paint the insides of gutters in the same way as the outsides, it is cheaper to use either black bitumastic paint or a thick coat of any gloss paint left over from previous jobs. Be sure to prime all bare areas before applying either paints. If you use bitumastic paint, keep a tough brush solely for this purpose; traces left in the bristles can seriously affect the colour and drying of oil paints.

Treat all downpipes (downspouts) as for gutters. If the pipes are black, check first if they are painted with bitumastic either by rubbing with white spirit, which will quickly dissolve bitumastic, or by applying a white undercoat to a small section to see if it becomes stained.

Pipes- painted with bitumastic will not take an oil-based finish unless you seal them first with stop-tar knotting or aluminium paint. Do this after the surface has been derusted, primed and washed with detergent. Apply undercoats direct to the sealer when it is thoroughly dry.

You can also paint plastic gutters and downpipes. These need to be cleaned thoroughly with strong detergent solution, scoured with fine ste<1 wool, dried, and painted with two coats of gloss paint. Gutters painted in this way then match metal ones.


Metal windows are usually made from galvanized steel and do not rust. But if they are very old and badly maintained, the zinc coating may fracture allowing the metal underneath to rust. In such cases remove the rust with medium grade steel wool or a flap-wheel drill attachment and prime immediately with a calcium plumbate or a proprietary zinc chro-mate metal primer.

Make sure that you remove all traces of steel wool before you prime, or they will rust under the paint and stain the gloss finish. If you use an electric drill with a flap wheel, wear safety goggles and make sure that the drill is correctly earthed.

If paint is flaking but the galvanized metal underneath is in good condition, this indicates that there is poor adhesion and all the paint should be removed. You can do this easily by dry scraping with a 25mm stripping knife or the flat side of a shave hook.

Use a spirit paint remover, suitable for use on painted metal, on the more stubborn areas. Most paint removers are rinsable, so a final wash with a detergent solution will remove all traces of the solvent and leave the metal clean for priming.

If there are no signs of rusting or flaking, wet abrade the old paint film with a grade 240 wet-or-dry paper. Put detergent in the water to make the rubbing down easier. When the surface is rinsed and dried, it is ready for undercoating.

Primed or washed and abraded surfaces need only one coat of undercoat and one coat of gloss. You can get a good straight line along the edge of the glass by using a well worn brush which ‘cuts in’ easily at the edges. Bring the paint about 1mm on to the pane to make a seal between the putty and the glass. Wash and clean the glass before you start painting.

Aluminium garage doors

If flaking is considerable—about 20 percent of the total area—strip off all the paint and start from scratch. Use a spirit paint remover as described above then, after washing and drying, prime the aluminium with zinc chro-mate—calcium plumbate is not suitable for aluminium.

If the paint is in good condition, use grade 240 wet-or-dry paper with detergent in the water to prepare the surface. On a large surface, such as a garage door, keep wiping the surface with a sponge so that the abrading paper does not become clogged.

On large, flat surfaces it is advisable to use a rubber or cork sanding block. Keep rubbing until the gloss of the old paint has all gone: the new paint will then key well.

One coat of undercoat and one of gloss is quite sufficient over primed or wet-abraded aluminium. If you are painting a large door, use a 50 or 75mm brush to get the best results, with a 25mm brush for getting into edges and grooves.

Timber surfaces

Exterior timber surfaces may be small in area—e.g. windows—or large—e.g. weatherboard or clapboard siding. Preparation is much the same, however. The existing paintwork will be in one of three conditions: Severe flaking: This usually leaves large areas of the substrate exposed so you need to remove the old coatings completely. You will get rid of most of the paint by dry scraping it with a 75mm stripping knife. Remove the tricky areas with spirit paint remover, protecting the areas which are not being treated with plastic sheets or paper. Wash off the paint remover with medium grade steel wool and detergent, rubbing in the direction of the grain.

When the surface is dry, dry abrade it with grade M2 glasspaper to smooth the grain which will have raised slightly by the action of spirit and water on the surface.

Coat any knots in the timber with shellac knotting, or they may later release resin which would stain the paint. You can apply knotting with a piece of rag. Make sure that the whole area of the knot is covered and overlap it about 3mm on to the surrounding surface of the wood. The surface can then be primed. Isolated areas of flaking: These are to be found most often around edges and joints. Scrape away the flaking paint with a stripping knife, shave hook or paint scraper until a hard, firmly-adhered edge of paint is left. Then use M2 or F2 glasspaper to dry abrade the exposed wood. Dust off and prime the wood, pushing the paint as deep as possible into the joints and overlapping the old paint by about 5mm all round.

Good condition: Paint in this state needs only to be wet abraded.

When the priming paint is dry, fill any holes or open joints so that no moisture can penetrate the wood and cause rotting. You could use linseed oil putty (mixed with a little undercoat to make it dry faster and be more flexible), mastic or caulking compound, or a proprietary filler, depending on the nature and size of the gap that you are filling.

Press the filler (or whatever) well into the gaps, and aim to leave a smooth surface as you work. With fillers, rub down smooth when hard.

Give timber surfaces which have been stripped at least three coats of paint over the primer to ensure good protection. This can be one undercoat and two coats of gloss or two undercoats and one coat of gloss.

Surfaces which have a covering of old paint in good condition need only one coat of undercoat and one of gloss.

All undercoats and glosses used on the outside of the house must have an oil base. The same undercoat and gloss paints can be applied to all primed or painted timber, metal or plastic surfaces; only the primer varies from material to material so that a good base is provided for painting.


Unless you require a special finish, you can treat and paint front and rear doors in the same way as timber windows. Doors, being larger areas, show up surface and painting defects more clearly, so you should take great care when preparing and painting them.

Because they are less exposed, doors rarely get into such a bad state that they need stripping. In any case, the extra thickness of old paint coats in good condition provides better protection for the timber and a smooth, hard foundation for high quality finishes. When the paint is completely stripped off a door it takes some time to build up a good enough finish with several coats.

You may have to strip your door completely if it is severely blistering or flaking, or if the paint coats already on it are too thick, making the door difficult to shut.

Whatever method of preparation you use, always remove the door furniture—handle, letterbox, key escutcheon—before starting work. This saves time ‘cutting in’ around them and produces a better finish.

The easiest, quickest and cheapest method of stripping the door is to burn off the paint with a blowlamp. Start on the mouldings, using a combination head shave hook to scrape away the peeling debris. Strip the fiat areas with a 50 or 75rrrm stripping knife, working behind the flame torch and always pushing the knife in the direction of the grain. When the paint is completely stripped, prepare and paint the wood in the same way as you would timber window frames.

If the paint is not to be stripped, you should wet abrade the door. For paint coatings with very coarse brush-marks and deep chippings use a grade 180 wet-or-dry paper. If the paint has a reasonable finish, a 240 grade paper will be sufficient.

Keep the surface wet and clean with a sponge dipped in the detergent water, rubbing the paintwork until all the gloss is removed and all the hard edges are erased. This may take a long time with regular changes of water and paper, but the effort is worthwhile. Keep a dust sheet or wad of newspapers under the door during the entire preparation and painting process: these absorb water and debris, and prevent dust being picked up during painting.

Apply the undercoat laying off very lightly with the tip of the brush in the direction of the grain to avoid brush-marks. When the undercoat is dry, abrade with a grade 1 glasspaper to remove any uneveness.

For a really smooth finish, fill the whole surface at this stage with either oil-based paste filler or vinyl-based fine surface filler.

If you cannot get the filler smooth with the knife, dry abrade it with Fl glasspaper. Use this method also between primer and undercoat for a door which has been stripped. A second coat of undercoat will seal the absorbency of the filler and provide a surface of even colour, but take great care when laying off the brush strokes. Two coats of undercoat will also ensure that any old colour will not show through.

Before applying the gloss paint, wipe over the surface with a tacky duster and lay clean newspapers under the door.

Remember to include the top edge of the door and, if possible, the bottom edge. If the door opens into the hall, paint the hinge end in the same colour as the outside. To achieve a glass-like finish, apply a second coat of gloss as soon as the first is hard. When the entire job is finished, you can replace any fittings.

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