Almost all metal surfaces will need priming before further decoration can take place. Metal primers may be oil or water based and will often contain additives to protect against corrosion.
For ferrous metals a single coat of primer will rarely be sufficient to provide adequate protection so two coats should be applied wherever possible. At the very least apply two coats to vulnerable or exposed areas such as external angles and fixing points.
Types of Metal Primer
Pre-treatment Primer or Etching Primer – used to improve the adhesion of paint systems to non-ferrous metals and surface coatings such as zinc galvanised metal. 2-pack paints provide a better result but can be expensive and less convenient to use. Adhesion promoters are another form of water based preparatory primers for non-ferrous metals and stainless steel with low VOC’s. A further application of a regular primer will usually be necessary to provide full protection.
Mordant solution is an alternative water based preparation for galvanised or non-ferrous metals. The solution causes a chemical reaction with the surface to provide a key for regular primers.
Zinc phosphate is a solvent based primer, less toxic than some other industrial primers, neutral in colour, often used as a general purpose primer for ferrous metals including iron and steel.
Chromate primers are solvent based, fast drying and suitable for use on ferrous and non-ferrous metals.
Zinc-rich primers – two-pack epoxy resin based for specialist use on blast cleaned steel. Also available as single pack solvent based primer and used for touching-up damaged zinc coatings. Usually applied by spray.
Micaceous iron oxide – an industrial oil based primer formulated to provide a greater film thickness for areas subject to high wear or extreme exposure.
Red Lead Primer – was used extensively for priming metal work but due to the hazardous nature of paints with a high lead content its use is restricted to industrial applications such as structural and marine steel work. It can also be used under licence in some circumstances for restoration work to ancient buildings.
Calcium Plumbate – another lead based primer with restricted usage. Commonly applied to zinc galvanised metal work. Available as an oil based single or two-pack epoxy based primer.
Two pack epoxy primer – Solvent-based with aluminium pigment, suitable for application not only to manually prepared steel surfaces, but also over existing, sound paint work.
Universal primer – All purpose primer typically oil or water based acrylic with white or light-coloured pigments. e.g. zinc phosphate. Convenient for small-scale maintenance work or spot-priming a variety of surfaces, e.g. wood, metal and dry plaster.
For new and large-scale work, primers which have been formulated for a specific purpose are preferable to universal primers.
Reference and Further Reading
BS 6150:2006 Painting of buildings – Code of practice Section: 8.3 Iron and steel and 8.4 Non-ferrous metals and metallic coatings.
BS EN ISO 12944 Corrosion protection of steel structures by protective paint systems for steel structures.
BS 5493 Code of practice for protective coating of iron and steel structures against corrosion.
BS 2523 Lead-based priming paints.