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.
Janice – Can you advise on zinc downpipes? My builder says he will have a primer for zinc which is great and asked us to get paint specially formulated for zinc for the topcoat. READ MORE…
Zinc is naturally resistant to oxidization and, therefore, does not require a specialist primer. What it does need though is to be properly degreased prior to painting and it may also need an etching-primer in order for any coating to stick to the surface. (Etching primer is, basically, an acid-based solution which takes away the shiny finish from surfaces such as zinc and galvanised steel).
The alternative is to leave the surface untreated for a year in order for it to ‘weather’. In other words, the natural process of being rained on repeatedly will denature the finish to such a degree that no further preparation is needed.
Once prepared (or weathered), I would use a general all-purpose solvent based metal primer and two coats of solvent based undercoat, finished with one or two coats of solvent based gloss. (*solvent based = oil-based).
If you are paying someone to do this for you then ensure you receive a written specification before any work is carried out.
Anon – Could you help me please? What spot primer should I use to redecorate a zinc canopy? It is approx. 25 metres long and forms a roof for a veranda. It was painted white by the previous owners. READ MORE…
A general purpose primer should do the trick since you only need to worry about adhesion, not protection as such.
Darren – We got steal dipped steal work what primer should be used on it? READ MORE…
For galvanised steel the traditional primer to use is Calcium Plumbate although some water based alternatives such as ‘Dulux Metalshield Quick Drying Metal Primer’ can work just as well.
Newly galvanised metal does pose a problem with painting though because the surface is often too smooth for paint to key to. Acid based etching primers can be used, if it’s absolutely necessary, although allowing the surface to weather over time can be just as effective.
Reina – Hi, can u tell me what primer to use in Crittall door panel? There is some metal showing? Thanks for any advice. READ MORE…
I am not familiar with Critall’s products so you need to contact them for more specific advice, although an oil-based general-purpose primer should cover most scenarios.