Features & reviews
Preparation, preparation, preparation: no shortcuts to a professional finish
August 2010
When it comes to applying paints and other coatings or treatments to wood, it's tempting to dismiss preparation as an over-hyped part of the process. Not so, says Richard Bradley, Marketing Manager Woodcare at ICI Paints AkzoNobel - taking shortcuts at this stage is the fastest route to all manner of painting nasties - including flaking, peeling and bubbling - and ultimately, a sub-standard finish.
Without proper care and attention at the pre-paint stage, the performance and longevity of a coating will be undermined. In fact, the vast majority of timber problems could have been prevented by taking a little more care before the coating is applied. But why is it so important and what steps should be taken to avoid the pitfalls of poor preparation?
In order to stay put, paint needs a stable base to which it can stick. Poor adhesion is the root of a large proportion of coating failure issues, so much of the preparation process is aimed at making the surface as 'sticky' as possible for the coating.
Grease and dirt are enemies of adhesion, so wood that has been exposed (eg in a kitchen, where cooking causes airborne grease to settle on surfaces easily), must be cleaned thoroughly before painting. In most cases, a solution of sugar soap or other mild detergent will do the trick. Some wood species, such as teak, are inherently very oily in nature, so before painting or staining should be thoroughly wiped down with white spirit and allowed to dry.
For exterior woodwork, fungal and algal growth must also be removed prior to painting. A fungicidal cleaner is recommended. Don't be tempted to skip this step if the coating itself has anti-fungal properties - this is to prevent growth, not treat microbes already living on the wood. Softwoods in particular are more prone to decay and staining by fungi so consider treatment with a wood preservative or preservative primer prior to a top coat of paint.
In the majority of cases, sanding will be required at some point in the preparation process. As soon as a tree is machined, it becomes vulnerable to the effects of the elements - particularly moisture, the sun (UV) and the action of micro-organisms such as mould and fungi. These elements can break down the 'glue' that holds the microscopic fibres of wood together, causing them to begin to detach from the surface, so it is vital that this damaged layer of wood is removed, revealing the sound wood underneath before painting.
In addition to removing any wood affected by weathering (whether this is visible to the naked eye or not), sanding will provide a smooth surface that will give a long lasting professional finish to woodwork.
Whether you use an electric sander for larger areas, or sandpaper for smaller areas, working with the grain will prevent scratching and is particularly important if you are using a transparent coating such as a varnish (or a semi-transparent coating, such as a stain). Most people are familiar with the idea of wrapping a piece of sandpaper around a block to sand a flat surface, but for more awkward shaping, try wrapping it round a firm sponge to allow it to get into the contours better.
After sanding, check the surface for holes and indents. Contrary to the belief of some tradesmen, nail heads and other holes will not get properly covered by a coat of paint, so fill them using either an all purpose filler or a specialist wood filler (for example, Polyfilla All Purpose or Polyfilla All Purpose Woodflex from Polycell Trade).
Bare/naked timber is quite porous, so the next step after sanding is to 'prime' the wood. The primer is a thin paint solution which soaks into the grain slightly to make the surface more adhesive. Without the primer, the paint won't stick as well, so will quickly flake and peel. If there are any knots in the timber, this is a good time to treat them with knotting solution, as this will stop any sap leaking out and spoiling the finish.
If a surface has been painted previously, it is not always necessary to remove it before re-painting. If it is in good condition (ie no sign of flaking or peeling), it will need a clean to make sure that it is free from grease and dirt, and a mild sanding down to give the surface some purchase for the new paint to stick to. A mask is recommended for sanding existing paintwork because older paints may contain lead and breathing this in can be dangerous. The safest way is to use wet and dry paper and keep it wet to minimise paint dust.
Wood preservatives such as Weathershield Aquatech Preservative Basecoat+ should not be applied over existing paint, stains or varnish, however, as the active ingredients will be unable to penetrate the coating - this sort of product should therefore always be applied directly to bare wood.
With the sanding, filling and priming out of the way, you should have the perfect surface to accept your coating of choice. Putting in the groundwork at the preparation stage will ensure that the resulting paintwork gives the timber long lasting professional finish that protects the wood and looks freshly painted for longer.
More information about choosing the right products for preparing and coating wood can be found at www.duluxtrade.co.uk, or call 0870 272 1100 and speak to a member of the Dulux Trade support team.
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