How to paint a Galvanised Roof

What Is galvanised iron?

Galvanised iron is steel that has been dipped into molten zinc metal. This increases the corrosion resistance of the steel. The resulting material has a bright silver colour and is highly resistant to red rusting. Old galvanised iron may lose the protective zinc coating and red rusting may then occur.  The surface of zinc metal oxidises rapidly to form a protective carbonate/oxide layer, which protects the zinc from further damage. This layer causes most paint failures because it can be relatively weak. This results in paint flaking. Zinc metal also inhibits the curing of enamel paints. This is the main cause of poor adhesion of enamels applied directly to galvanised iron without the correct primer. Waterborne acrylic paints such as Homestead Roof Paint adhere well to clean galvanised iron and have superior exterior durability compared to enamels. However, galvanised iron is prone to “white rusting” in humid or corrosive environments and a properly designed anti-corrosive primer Metal Primer (Waterbased) is necessary for long-term durability.


Surface Preparation:

This is the critical part of any painting project: poor preparation is the principal cause of coating failure. It is very important to remove surface dirt, grease and other contaminants as well as loose paint coatings and surface oxidation (e.g. dull, matt grey surfaces) in order to ensure optimum adhesion and durability of the paint system. The necessary steps are as follows:

  1. Wash the surface thoroughly using water and a strong brush. This should remove all loosely bound contaminants and  any poorly adhering paint coatings. Use a spatula or flat blade to check adhesion of any existing paint coatings. A high-pressure water jet is recommended.
  2. Apply a quality Etch Cleaner to any exposed metal. This dissolves surface oxidation and helps to remove any oily contamination. Leave in contact with the roof for 5-10 minutes maximum, then rinse the roof with clean water. Leave it to dry out. Apply the primer within 2 hours of cleaning to avoid possible re-contamination of the surface.
  3. Check the adhesion of any existing paint coatings by pressing adhesive tape (e.g. masking tape) onto the dry surface.
  4. Pull the tape off the surface and inspect the underside of the tape.
  5. No coating removed; no dirt or chalky material on the tape:  Surface is suitable for painting
  6. Powdery layer removed (painted surfaces only) Paint surface is chalky: apply Fired Earth Bonding Liquid.
  7. Sandy particles removed
  8. Surface is dirty: rewash the surface.
  9. Paint coating removed
  10. Surface under the coating is weak: strip the paint film and repeat the cleaning process.
  11. Prime exposed metal using anti-corrosive primer Metal Primer (Water Based) for galvanised steel and leave to cure for 2-4  hours before continuing. If red-rusting is present, then use an appropriate anti-corrosive primer  Metal Etch Primer (Solvent Based).

Paint application

  1. IBR or corrugated roofing: Apply primer or Fired Earth Extreme Roof to the prepared surface using a roller (wool or Synthetic  type); brush or by airless spraying.  Two coats are recommended for optimum durability and   quality of finish. Allow the first coat to cure for at least 2 hours before applying the final coat.
  2. Gutters or downpipes: Apply primer orFired Earth Extreme Roof to the prepared surface by brushing. A small fine foam roller may also be used.

Calculating the amount of paint required

  1. Estimate the plan area of the roof by measuring the length and breadth of the walls.
  2. Multiply this area by 1.25, multiplied by 2 for two coats.
  3. Estimate the paint quantity needed by dividing the above area by coverage rate of the product type.
  4. One litre of paint will cover approximately 30m of 30mm radius down pipe or gutter. This should produce a slight excess of paint, which is useful for touch-up work as required.

Other important information

  1. NEVER apply paint if rain is forecast, or expected, during the day of application
  2. Apply paint after 8h00 (summer) or 10h00 (winter) to ensure the roof is not too cold or damp.
  3. It is recommended that the paint is applied before 12h00 to avoid excessive roof temperatures and to allow the paint to harden sufficiently before any afternoon rain may occur.
  4. Always use the SAME batch number for touch-up work. This reduces colour/sheen variations, which otherwise may be visible on the painted roof.

Description of products described above

  1. Metal Etch Primer (Galvanised): A specially formulated corrosion-inhibiting acrylic primer for galvanised iron in good condition. (Anti-Corrosive type).
  2. Metal Etch Primer: A primer for red rusted galvanised iron surfaces. (Anti-Corrosive type).
  3. Fired Earth Extreme Roof acrylic is a high quality acrylic, medium sheen finish paint with outstanding weathering and alkali resistance properties. It can be applied onto suitably prepared fibre cement tiles, roof sheet, clay or cement roof tiles and pre-primed galvanised iron roofs.Available in the following colours: Black, Grey, Charcoal,  Red, Brown, Terracotta and Green.

Other important information

  1. NEVER apply paint if rain is forecast, or expected, during the day of application
  2. Apply paint after 8h00 (summer) or 10h00 (winter) to ensure the surface  is not too cold or damp.
  3. It is recommended that the paint is applied before 12h00 to avoid excessive temperatures and to allow the paint to harden sufficiently before any afternoon rain may occur.
  4. Always use the SAME batch number for touch-up work. This reduces colour/sheen variations, which otherwise may be visible on the surface being painted.