Durability of any finish is highly dependent on proper application, which includes good preparation of the surface to be coated. Specific details on surface preparation depend on what condition the wood is to begin with – read on for tips that apply to various scenarios.
Surface Preparation for Fresh Wood
While fresh, clean wood can be coated without surface preparation, a light sanding with 100 grit sandpaper (and dust removal) can double the service life of some water-based coatings. For best results apply a coating to a fresh wood surface as soon as possible after planing or sanding. If exposed to rain and sun for more than two weeks, adhesion of coatings will not be as good. The surface must also be free of anything that will interfere with coating adhesion, such as dirt, damaged wood fibres and moisture. Grade stamps on wood should also be removed before applying a semitransparent stain, preferably by sanding.
If there are discolourations caused by dirt, iron stains or other discolourations on the wood surface, cleaning may be desired. It is always preferable to achieve cleaning with sanding when possible. Another safe way to clean wood without damaging the surface is to simply use a garden hose, with or without a pressure nozzle. Use pressure-washing only with extreme care as it can damage wood, especially low-density species such as western red cedar. The pressure should be kept at a minimum, and never hold the nozzle in one place for a long time. If necessary, use a little bit of dish detergent, and lightly scrub (not with steel wool, as this will leave iron stains) in the direction of the grain for any stubborn discolourations. For discolourations that resist soap-and-water cleaning, chemical cleaners will be effective. The chemicals in commercial wood cleaners can be caustic soda (sodium hydroxide), sodium metasilicate, oxalic acid, citric acid, phosphoric acid, borax or some mixture. Wood cleaners containing caustic soda at a 1% – 2% solution will remove nearly all discolourations with the least damage to wood. Some acid cleaners are especially effective for removing extractive stains and iron stain. Bleach is commonly used for cleaning wood, but we do not recommend this, since a poor wood substrate will usually be left behind for subsequent coating. Resin (pine pitch) can be generally removed with mineral spirits. Please note that all acidic or alkaline chemicals need to be thoroughly rinsed off before coating. Chemicals can be toxic, corrosive and harmful, so handle all these chemicals with care and follow all manufacturer’s instructions.
Surface Preparation for Aged Wood
Wood coatings need a fresh surface or the coating simply won’t last. The longer wood has been allowed to weather, the poorer the coating adhesion. If a fresh surface is allowed to weather or age outdoors for more than two weeks, coating adhesion will deteriorate. This is mainly due to wood damage from sunlight. Weathered wood surfaces usually have a higher acidity, higher contact angle, and lower surface energy.
Restoring an aged wood surface is necessary before applying a coating. The damaged (aged/weathered) wood fibres must be removed, exposing fresh wood. Also, any discolourations will typically be removed along with the damaged fibres, so the process of restoration is simultaneously a cleaning process. Wood restoration can be achieved with sanding or with chemicals, but sanding is always preferable when possible. Sanding can be done by hand or machine until the true wood colour shows. Then brush off the sawdust and apply the coating immediately. For many jobs, a chemical method will be far easier. Read the label of each product to identify the active components. In general, caustic soda (sodium hydroxide) is the best chemical choice for both cleaning and restoration. It effectively removes weathered wood fibres from the surface and leaves the surface at a suitable pH for coating. Oxalic acid is also commonly identified as a wood restorer, however, it is only effective at discolouration removal and does not remove the damaged wood fibres from the surface – in other words, it is not restoring the wood to be an appropriate substrate for a coating. However, oxalic acid can be used to return the original wood colour after the use of sodium hydroxide. Sodium hydroxide will slightly darken the wood, and, if this is undesirable, simply rinse the wood with oxalic acid after restoration with sodium hydroxide. Please note that all these chemicals must be handled with care and all manufacturer’s instructions should be followed, as the chemicals can be toxic, corrosive and harmful. Where the wood is close to plants, wet down the leaves with a garden hose prior to and after chemical use. Wood surfaces should also be thoroughly rinsed with water before coating.
Maintaining a coating means giving it a wash occasionally, watching for signs that the coating is losing integrity, and applying a fresh coat before full failure sets in. If a coating is reapplied before the last coat has failed, the stripping process may not be necessary. It’s time to apply another coat when paint has worn down to the primer, or if the coating colour has undesirably faded, or if the surface of water-repellent treated wood no longer beads water. Then wash or brush off dirt and apply a new coat. Any areas showing failure (the coating has lifted from the surface or cracked, or bare wood is showing) can be spot-treated. Remove any loose pieces of paint and use sandpaper to feather the edges of adjacent sound paint so the transition won’t be evident through the new paint layer. Also sand away any weathered wood. For large scale failure, refinishing will be necessary. For all coating systems, there is a limit to the number of coats a surface can support. When the coating gets too thick, refinishing will also become necessary.
Refinishing a coating means stripping off the old coating and starting over. This is necessary when large areas of the coating have failed, or the coating is getting too thick for refinishing, or if a decision is made to change the type of coating. A coating has failed when it no longer adheres to the wood surface. If the coating has bubbled, cracked, or peeled, it must be removed. If the coating has simply faded but otherwise appears to still be well-bonded, it may not need to be removed. When a change of coating type is desired, the new coating may be incompatible with the old coating – to ensure a good bond for the new coating, strip off the old one. Remove coatings by sanding or with a chemical product. Sanding has advantages over chemical stripping in restoring the fresh wood surface, but even if sanding is done by machine, it is still very labour-intensive for large painted areas typical of outdoor projects. Sandblasting is not recommended except for large timbers and logs, as it will pit the wood and is hard to keep away from elements like window frames. Powerwashing will only remove loose paint, leaving behind paint that is still adhered. So, a chemical approach is generally regarded as the most effective and least labour-intensive way to strip a coating. Sodium hydroxide at a 6% – 8% dilution is the recommended chemical for stripping – and offers the additional benefits of cleaning discolourations and restoring the wood surface at the same time. Products containing sodium hydroxide are corrosive and should be prevented from touching skin. Follow manufacturers’ instructions. There are also other chemical products for stripping coatings in the market. After stripping with chemicals, always give the wood a final rinse with water. Many projects will still require some light sanding around stubborn stains or heavily damaged wood.