Painting exterior masonry is a fairly simple process of adequate preparation and selection of paint colour. The choice of paint is usually a matter of budget and/or personal bias because exterior masonry paints are all the same right?
Not really no.
There are water based paints, oil based masonry paints and mineral paints as well as a rubber based paint known as Pliolite. Factor in also the new ‘Eco Paints’ and you’d be right to be a little confused.
So which is best and does it really matter?
First of all the choice of paint will depend upon the type of wall surface and whether it has been painted before. Generally, the exterior walls of your property will be plain brick or stone or finished with some kind of cement based render – older properties (particularly those with a single thickness of wall) may have a lime based render finish.
Previously Painted Walls
For previously painted walls in good condition a standard water based masonry paint such as Dulux Weathershield should be perfectly adequate. If your budget doesn’t stretch that far then an own brand equivalent such as B&Q should be fine.
Most exterior wall paints are water based and contain some form of acrylic binder. They should be micro-porous – in other words, they allow any moisture trapped in the wall surface to evaporate.
Unpainted Cement Rendered Walls
If you have cavity walls and the render is in good, dry condition there is no reason why you shouldn’t use a conventional water based finish. You may need to do a little preparation such as filling any minor cracks and priming any loose or friable areas with stabilising solution.
Afterwards simply coat with two or three coats of masonry paint in your chosen colour (the first coat slightly thinned with water).
Stabilising Primer is ideal for chalky or powdery areas. It acts like an adhesive – providing a solid base for subsequent coats of paint to stick to.
However, it is NOT a suitable primer for ordinary rendered walls despite what your neighbour or bloke down the pub may tell you. A thinned coat of regular masonry paint is all that is needed as a primer for sound rendered walls and stonework.
For older properties with a single thickness of wall it will be necessary to ensure that the wall is able to breathe (in other words, the surface should be permeable and allow any trapped moisture to escape). Ideally the render will be lime based or with a low cement content finish.
In theory a quality water based system, such as Dulux Weathershield, will perform quite adequately and should last many years.
However, you may consider using a mineral based paint which could perform considerably better?
Mineral Based Exterior Paints
Put simply, mineral paints are made with a silicate binder which reacts with the surface of the wall to form a long-lasting bond, unlike traditional paints which simply form a skin. They are naturally alkaline, so inhibit mould growth and carbonation; mineral paints also have the added advantage of being eco-friendly too.
The disadvantage is that they require more thorough preparation, special primers and are, initially, much more expensive – although the increased performance should pay back over the long term.
Each have their own detailed specifications for different kinds of wall surface and these must be followed to the letter in order to enjoy the maximum performance.
Lime Rendered Walls
For old buildings with a lime based render or lime based pointing a traditional Limewash is ideal. Limewash is simply a paint made from lime powder and water together with a colourant.
The main advantages of limewash are they bind to the surface very well and are totally breathable – another benefit is that they are relatively inexpensive.
Care needs to be taken with application and sometimes additional binders need to be added to aid adhesion.The finished result can be very striking though – looking much more natural than a traditional paint finish.
Limewash paints are available from a number of sources including Ecos, Ty-Mawr, Little Greene and Farrow & Ball.
Technically, an ECO paint is simply a paint which has a very low or zero volume of Volatile Organic Compounds. While some ECO paints are obviously superior to others there isn’t any case for suggesting that an ECO paint will perform any better or worse than any other form of exterior paint – it will simply be a matter of personal choice.
It’s worth noting, however, that mineral and lime based finishes, mentioned above, are by their very nature eco-friendly – so would make a good choice if you wanted to be as environmentally friendly as possible?
A Little Preparation Will Go a Long Way
As always, the amount of time you spend preparing the surface will show in the end result and, if done properly, a painted exterior wall will last at least ten years before it needs re-coating.
Bear this in mind when making your choice of materials as it really isn’t worth skimping and a few extra pounds spent at the outset will go a long way.