Painting of a metal surface depends on good adhesion, and paint adheres best to surfaces that are well prepared, clean and free of contaminants. It is important to ensure the surface is in this condition before coating it, as the avoidance of important steps in surface preparation can cause even the highest quality of metal paints to fail.
Inadequate surface preparation is the number one cause of metal coating failures. Therefore, just as a building needs a solid foundation to last, so too must the metal start with a sound surface.
Iron and steel are two of the most commonly used metals, with both types of metal being subject to corrosion and rust. Rusting of metals can cause disintegration resulting in the weakness of the structure. To ensure the longevity of these metal surfaces, below are the recommendations for the protection of the two of the most common metal surfaces: ferrous and galvanised.
Ferrous metals contain or derive from iron that is commonly used in the manufacture of castings, fabricated sheet steel, and wrought iron. With the exception of stainless steel, all of these metals will rust, which can eat away at the metal as well as spoil its appearance and undermine any applied coatings.
Rusting can start almost immediately when the unprotected ferrous metal is exposed to the elements such as rain, snow, or moisture in any form. it is in your best interest to stop any rusting that has begun by keeping the moisture and air from interfacing with the metal after painting. Therefore, ferrous metals require very thorough surface preparation. Anything less than this will not only seriously compromise the integrity of the metal itself but also the appearance and durability of the finished paint job.
The importance of metal surface preparation
The first step to preparing a ferrous metal surface is to ensure that the surface for painting is noncorrosive by removing any loose rust and peeling paint. For smaller surfaces, using a chisel-style scraper will take off heavy rusting and loose paint then a hand-held wire brush will remove any of the rust residue left. However, it is not necessary to remove every bit of rust to take the surface down to the bare metal, but rather to remove as much rust as these tools can allow. On larger surfaces, power wire-brushing or disk sanding with aluminium oxide paper are effective methods. Whatever the chosen method, it is important to wear PPE (personal protective equipment), including eye protection and a good dust mask.
Note: Wire-brushing will leave surfaces with rust residue/small particles of loose rust and dust, simply brush these particles with a soft bristle brush followed with a thorough rinsing with clean water.
If you are working with a new iron or steel surface, surface preparation is equally as important. New ferrous metals often are covered in mill oil on it and in some cases, have small amounts of rusting that is not visible to the naked eye which will often result, in the premature failure of the paint job.
Good timing equals good priming
Once a ferrous metal surface is free from rust and other impurities, priming should be applied as quickly as possible. This is because rust can begin to re-form on iron or steel if the surface is exposed from as little as two days, causing you to prepare the surface again.
When painting ferrous metal, it is important to apply a high-quality metal primer as it needs to perform two important functions:
- Providing of the bond between the topcoat and the metal.
- Inhibit corrosion.
When priming ferrous metal:
- Use a quality exterior rust-inhibitive primer.
- Apply the primer at the recommended spread rate in order to achieve adequate film thickness to build corrosion resistance.
- Consider applying a second coat of primer for maximum corrosion resistance.
- DO NOT return unused material to the original container as rust particles picked up on the paintbrush will contaminate the paint in the container, reducing the shelf life.
Anti-Corrosion Primers work well on extremely rusty metals, maximising rust prevention and reducing the roughness of the substrate for a smoother top coat. work well in applications.
Selecting the right coating is important when painting ferrous metals. For exterior metals, a high-quality acrylate rubber paint is generally a good choice because due to its tough anti-corrosion protection, it can last as much as two to four times longer than conventional paints. It also has a quick drying time and high weathering properties.
Galvanised metal is iron or steel that has a thin coating of zinc on it to help prevent rusting. It is important that newly galvanised surfaces are clean, dry and free of contamination by washing it with a solvent cleaner and thoroughly rinse it before painting. This step is crucial as you need to clean off any zinc or residual oil left from the galvanising process, which can interfere with adhesion of the paint.
Take very great care if you are to coat newly applied galvanising. This must be thoroughly cleaned and abraded before coating as the new galvanising will act almost as a release agent.
Should the surface that is to be painted already have an existing coat on it, the performance of any new coating is going to be directly dependent upon the adhesion of the existing coating. If the existing coating is still well adhered and being re-coated for aesthetic purposes for instance, then one simply has to ensure the coating is cleaned and keyed sufficiently to ensure inter-coat adhesion. If however, the existing coat is flaking and/or peeling, it is strongly advised to remove the existing coating completely and have a ‘fresh start’.
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