The purpose of preparing a substrate for painting is to ensure that it will accept and retain the paint or stain with the minimum of interference from surface contaminants or from surface deterioration of the substrate itself.
The profile of the surface and its porosity will also influence adhesion of the paint system and its ultimate durability.
For new timber, the profile can vary from a smooth (planed) surface to a sawn textured surface. This will dictate the final appearance when coated.
The sawn or roughened (coarse sand papered) surface will give a more positive mechanical key to applied coatings but, in the case of a conventional paint system, will result in a much thinner film on the surface peaks which will deteriorate more rapidly unless additional coats are applied. Some surfaces are, however, ideally suited to opaque or semi-transparent stains.
As recommended by the paint manufacturers, surface contamination by dust, grease, oil, plaster or mortar, mould or fungi, brick-cleaning solutions, or other substances likely to affect paint adhesion, must be thoroughly cleaned/removed before priming. Prevention of such contamination during installation is better than subsequent cleaning.
A special consideration exists with buildings adjacent to the sea, where salt contamination of the surface occurs very rapidly and is not always visible. This must be washed off with fresh water shortly before the start of work, while still allowing time for the surface to dry. If prevailing winds are carrying salt laden air to the site, repeat washings will be required each day painting is to proceed.
For timber which has been exposed to the elements for a considerable time and has suffered the weather's oxidative effects on both lignin and cellulose in the surface layer, the timber should be sanded or dressed to a fresh surface before priming.
Any holes or depressions in the surface which may have occurred as a result of mechanical damage should be scraped clean, primed and filled with putty compatible with the paint system to be used.
Some timbers and wood based building boards are supplied pre-primed and provided they are not exposed to the elements for extended periods, may be finished with undercoat and an 100% acrylic paint without further priming (or as recommended by the manufacturer).
Pre-primed exterior cladding such as weather board comes with several pre-primed options depending on the manufacturer. It can be available with one or two coats of machine applied alkyd primer, or has also begun to be introduced with two coats of new technology acrylic primer.
Each option requires a slightly different approach to painting but in general prime bare end grain and any exposed wood is required as soon as possible.
The preparation for repainting a previously painted surface will depend largely on the condition of the surface. If the paintwork is flaking, blistering, cracking or checking, it will be necessary to remove either the entire film back to the substrate or at least enough of it to provide a sound base for subsequent coats.
If the surface is chalking, or has surface mould present, manufacturer’s directions for removal should be followed.
If for some reason a pre-primed weather board has been left exposed to the weather for extended periods, removal of the chalking/oxidation deposits of the primer and the application of a further full coat of primer is required.