All paint coatings are applied to a substrate to produce a paint system, e.g. substrate primer, undercoat, topcoat. Whether the overall result is satisfactory will depend on how these layers adhere together. How they perform individually and where-the weakest link is. Sometimes the weakest link is at the substrate-primer interface (e.g. poor adhesion), or in the substrate itself (e.g. wood suffering from wet rot). For this reason, it is necessary now to consider the substrate itself and its particular reactions. There are numerous reasons why it may be necessary to improve the substrate. It may be poorly produced e.g. too rough a finish, it may have a natural weakness e.g. chipboard is too porous, it may have been damaged or contaminated e.g. dented or greasy metal, it may have deteriorated during storage e.g. rusty steel. Many of these problems can be dealt with by physically treating the substrate to produce a clean level sound surface suitable for painting. Such treatments include the planning and sanding of wood, the degreasing and sand/shot blasting of steel and the filling and sealing of damaged or porous surfaces. Although not used to a large extent in the refinish area, wood is one of the most commonly painted surfaces. All surfaces are continually exposed to bacteria and the spores of fungi which are particularly destructive to wood. Brown rot is so called because it attacks only the cellulosic material in wood, leaving behind lignin which is brown in colour commonly known as 'dry rot'. White rot, on the other hand, destroys both cellulose and lignin and is known as white rot or ‘wet rot'. Other fungi cause surface disfigurement known as blue stain or sap stain. Perfectly sound-looking wood is almost sure to have spores on its surface and there may even have been penetration into the cells below, covering these organisms with paint will not prevent trouble later. This is especially so if design deficiencies in the wood structure aid penetration of water, encouraging the moisture content of the wood to raise above 20%, the level necessary for the fungi which cause rot to become active growing and multiplying under the coating, feeding on the wood. Fungi can be prevented from attacking wood by treating it chemically and keeping its moisture content to a minimum, active ingredients include pentachlorophenol tributyltin, oxides of copper and zinc naphthenates or mixtures of copper sulphate and sodium or potassium dichromate. These act in a variety of ways to destroy or inhibit the growth of fungi. It is also important to get these preservatives deep into the wood the best techniques use vacuum or pressure to displace air from the cells and force preservatives into them. Special low viscosity preservative containing primers may add markedly to the life of an exterior paint system by destroying organisms on or in the surface layers of the wood and also by penetrating open joints to seal end-grain, thus helping to keep the moisture content of the wood as low as possible.
