The primer must adhere well to the substrate, offer protection from deterioration or corrosion and provide a good base for the undercoat. To ensure adhesion, the substrate surface must be free of loose or degraded material. Appropriate systems are indicated in Table 17.1. For use on timber, primers may be oils, alkyd resins or acrylic emulsions, frequently with titanium oxide. Aluminium wood primer is recommended for resinous woods and to seal aged creosoted and bitumen-coated surfaces. For the corrosion protection of ferrous metals, primers incorporate zinc or lead-rich compounds within oils or alkyd resins. While lead-based paints such as red lead and calcium plumbate are considered environmentally less acceptable than the alternatives, they remain very efficient in the inhibition of steel corrosion. The newly developed low-VOC coatings offer temporary protection against the corrosion of structural steelwork either as prefabrication or post-fabrication primers. Alternatively, acrylated rubber paints, which form a physical barrier over steel, may be used as primers. For non-ferrous metals, zinc phosphate primers are frequently used. The application of primers suitable to ferrous metals may cause increased corrosion on non-ferrous substrates, particularly aluminium. Masonry paints are usually based on alkyd or acrylic resins with titanium oxide; where surfaces are likely to be alkaline, such as new plaster, brickwork or concrete, alkali-resisting primer should be used.

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