EXTERIOR WALL PAINTS

Brightening up the house, matching in a new extension, covering up rendered brickwork — exterior paint can work wonders on walls.

But which paint is right for which surface?

And, what are the differences between the types available? Check here before you buy.

Cement paint

This is the cheapest of all outdoor paints and, in spite of its name (which refers to what it’s made of. Plus accelerators and waterproofing agents) it’s suitable for most types of previously unpainted brick, concrete and cement rendering, pebbledash and asbestos sheeting. Porous surfaces should first be damped down (this stops the paint drying out too quickly) and the paint applied while the surface is damp. For this reason it is especially good in damp or humid conditions.

Cement paint is sold by weight as a powder that you mix with water to a creamy consistency before applying it with a fibre brush. (Don’t use a bristle brush — it would be destroyed by the alkali-resisting pigments used in the pastel shades.) Brushes can be cleaned in water, which makes up somewhat for the inconvenience of mixing, or a spray gun can be used. Its coverage is good — applying 2 coats, you can expect to paint between 15 and 30 sq metres with a 10kg pack depending on the roughness of the wall — and it dries in 2-4 hours.

Cement paint is not very durable, and turns powdery in time. The choice of colours is limited and it is mainly sold by builders’ merchants rather than decorating stores. If you want to paint over a cement paint, an alkali-resisting primer/sealer must first be applied.

What is whitewash?

Yesterday’s answer to masonry painting and now not sold commercially, whitewash was powdered limestone and chalk mixed with water and applied with a brush. It cost little, spread well on dry surfaces and could eaily be removed just by washing it down. In fact, the weather usually did the job without any help! Modern paints are made to last and are therefore more expensive, but as a low-cost operation you could buy the components and make it yourself.

Exterior gloss flows well and can easily be applied by brush, rolleror spray gun. It will be tack-free in 4-6 hours, though it should be left overnight to dry before a second coat is applied. The cleaning solvent is turps and the coverage on a suitably prepared wall should be between 15 and 17 sq metres per litre. Nevertheless, it’s quite expensive when you add in the cost of all-surface primer and undercoat that are needed.

Exterior grade emulsion

Also known as plastic paint, this is the weather-resistant version of emulsion used indoors. Based on acrylic polymers, which give excellent adhesion, it’s easily applied with a brush, roller or spray gun. Equipment can be cleaned in water — but do it immediately after use. It’s suitable for most surfaces — except those painted with masonry paint — and it gives a good seal to brickwork, cement rendering, stucco and asbestos. Emulsion dries very quickly — within 4 to 6 hours — but perhaps most important, it’s fairly inexpensive. How much it covers obviously depends on the porosity of the wall, but 8-10 sq metres per litre is usual.

Stone paint (or textured paint)

The name doesn’t mean you can use it only on stone, for in fact the paint is similar to exterior grade emulsion but gives a textured finish.

• Don’t paint indirect sunlight as it can cause the pigment colour to change. It’s best to paint areas that have been dried by the sun and have just fallen into shade. By the time the sun strikes them again the next day the paint will have dried.

The resins on which it’s based are more robust and to help it cope with cracks, holes and natural movement in the building. it’s ‘beefed up’ with various fibres and mineral powders (such as granite, stone or rock). It adheres well and is weather-resistant. Its major drawback is the coverage — on average about 5 sq metres per litre, though you can improve on this by applying an undercoat of cement paint.

Only masonry paint outdoes it for durability, but stone paint has the added advantage of one of the best colour ranges. You brush on a thickish coat, then ‘work’ the surface with a fine rubber stippler (or the brush) to create the effect you want. This paint can be difficult to brush on if you don’t have an artistic flair, but you can use a roller or spray gun (this must be fitted with a modified nozzle to cope with the added fillers).

Masonry paint

A durable, oil-based paint producing a smooth semi-gloss finish, this is the most expensive of the outdoor finishes but can be used on brick, stucco, concrete, cement rendering and asbestos sheeting. It’s also the one with the most drawbacks. Although it can be applied quickly with a brush. Roller or spray gun, you first have to prime the wall, then wait at least 12 hours for the first coat to dry and finally clean your equipment in turps/white spirit.

Using a spray gun

When using a spray gun move it at an even rate over the surface — if you move it too fast the paint will break up into droplets rather than creating the necessary continuous wet film. If the gun is held too far from the surface the solvent will evaporate before the paint reaches the wall. If hiring a spray gun. Tell the supplier the type of paint you’re using so you can get one that’s up to the job. And don’t work on a windy day!

The colours offered are mostly pastel shades. As for coverage, on a smooth surface (and you need a very smooth surface for the best finish) you should get up to 15 sq metres per litre. It is extremely durable, and is resistant to airborne dirt and pollution.

Is tough and flexible and has additives such as mica or nylon fibres to give reinforcement and texture. It both protects and decorates new and old cement rendering, concrete, brickwork, pebbledash, stucco and asbestos sheeting. Apply by brush, roller or spray gun. Although water is the solvent, it is often difficult to remove all fibrous material from brushes or rollers and these cannot be used again for a finishing coat. The coverage is about 7 sq metres per litre on a non-porous wall, less if the surface is very porous.

Primers

These are only ever needed on masonry if the surface is defective and would prevent the paint adhering. Will need a stabilising solution — a resin-based, treaclelike, thin liquid primer which soaks up the powder and seals the surface. Should have all loose material scraped off and a plaster-based sealer applied which will fill in to a certain extent and cement down the edges of the previous surface.

– Don’t paint when rain is likely or when there’s damp or dew around — the slightest drop of water will permanently spot drying paint.

– Large brushes or rollers, instead of speeding up the job. Tire you out sooner.

COPING WITH FAULTY SURFACES

Surface preparation

Exterior paint can only give the best finish if the surface has been made good. Water is the number 1 enemy of outdoor surfaces. Driving rain and frost break them down making them dusty and absorbent; moisture will get into cracks and thus behind the paint film; dripping water from leaky pipes or even nearby trees will encourage organic growth. If the weather has eroded the surface in any way you should.

– scrub or scrape off any loose material, organic growth

– apply a stabiliser or waterproofer.

– fill cracks with outdoor-grade filler.

– leave the surface to dry completely before decorating.

Flaking occurs when paint has been applied to an unstable surface. Scrape or rub off with a stiff bristle brush, make good, apply stabiliser.

Iron staining is caused by pyrites in the pebble dashing. Prise off offending pebbles, cover up stains with aluminium leafing primer.

Brittle and crazed paint on roughcast concrete occurs through moisture penetration and frost action. Stabilise, repair gaps by stippling on rendering with stiff bristle brush.

Common brick blistering happens because the surface wasn’t dry when painted or because of water penetration after. Scrape off fraking paint, then apply a waterproofing solution.

Organic growth is found on surfaces which retain moisture. All visible signs must be removed with a stiff bristle brush, then a fungicide applied. Wash down after 24 hours.

Paint film failure occurs where the filler was unsuitable. Rake out the old, remove all flakiness, repair with exterior grade filler. To even it out the area might need a textured finish.

Movement cracks should be stabilised and filled with exterior filler. Brush on bitumen waterproofer, embed glass fibre strip, cover with more bitumen and seal with aluminium spirit-based sealer.

Chalkiness indicates an unstable surface. Remove with a stiff bristle brush, then apply a stabiliser— do the hand test again before painting.

Sorry, comments are closed for this post.