General.
New Galvanised steel is one of the most difficult substrates to paint, therefore, correct surface preparation is paramount.
Galvanised surfaces will last for many years untreated and when exposed to the elements the bright finish will lose its brightness and tone down to a matt grey finish.
If weathering has taken place then the pre-treatment of the surface before painting may be different from that stated below.
Paint manufacturers offer different systems for painting galvanised steel and other non ferrous metals, therefore, they should be contacted and their recommendations strictly followed.
The following information is generic and for guidance only.
System and products,
Preparation.
Degrease with a suitable degreasing agent, methylated spirit or other cleaning solvents. For small areas hand etching with a suitable abrasive paper or cloth will improve the surface key. For large areas sweep blasting the surface to obtain a fine textured profile will provide a good surface for subsequent painting.
Mordant solution (T-Wash) should be applied to the prepared surface which on drying turns the bright metal surface black confirming the surface is clean and free from all contaminates.
Pre-treatment of galvanised surfaces
Priming
Products depending on manufacturer.
Priming coats can be either PVB Self etch primer, Zinc phosphate or general purpose primers. Undercoats and finishing materials can be oil or water based along with high performance epoxy and polyurethane protective coatings. Fire protection finishes are also specified.
Once the performance requirements have been decided, the manufacturers product data sheet should be obtained and instructions followed.
Note: PVB Self etch primers are susceptible to moisture and should be overcoated as soon as possible without exposure to high humidity or rain. Also the dry paint film is translucent so it is important to ensure the entire surface is coated.
In cases where painting of galvanised steelwork is required then steel fabricators often apply a holding primer before the steel is delivered to site. Under these circumstances it is important to establish what product had been used and consult the manufacturer for a recommended specification.
For further information on Zinc coated steel contact: British Galvanizers Association. ‘What is Galvanizing-See:> https://www.galvanizing.org.uk/galvanizing-process/
Product illustrations provided by Bradite Limited. http://www.bradite.com