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 and results in a material that has a bright silver colour and is highly resistant to red rusting. Galvanised iron can over time lose this protective zinc coating and red rusting can occur. Zinc metal surface oxidises rapidly to form a protective carbonate / oxide layer, which protects the zinc from further damage. As this layer can be relatively weak it causes most paint failures, resulting in flaking.
Zinc metal also inhibits the curing of enamel paints. This is the main reason enamels adhere poorly when applied directly to galvanised iron without using the correct primer. Waterborne acrylic paints, such as Promac Roof Paint, adhere well to clean galvanised iron and offer superior exterior durability compared to enamel paints. However, galvanised iron is prone to “white rusting” in humid or corrosive environments and a properly designed anti-corrosive primer, such as RB10 Metal Primer (water-based), is necessary for long-term durability.
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 coatings of paint and surface oxidation (e.g. dull, matt, grey surfaces) in order to ensure optimum adhesion and durability of the paint system. The necessary and recommended 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. Use of 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 two hours of cleaning to avoid possible re-contamination of the surface.
3. Conduct an adhesion check of any existing paint coatings by pressing adhesive tape (e.g. masking tape) onto the dry surface. Pull the tape off the
surface and inspect the underside of the tape.
• If no coating has come off, and there is no dirt or chalky material visible on the tape then the surface is suitable for painting. *
• If the surface is chalky or a powdery layer is visible on the tape apply a quality Bonding Liquid.
• If sandy particles are removed the surface is friable and the surface must be rewashed.**
• If the paint coating comes off, the surface under the coating is weak and should be stripped, and the cleaning process repeated.
check relevance to galv steel
* Check the porosity of the surface before painting by applying a droplet of water. If it is easily absorbed into the tile then the application of a quality
Bonding Liquid, to seal any residual particles, is recommended.
** As it may not be possible to remove all sand particles the application of a quality Bonding Liquid, to seal any residual particles, is recommended.
4. Prime exposed metal using anti-corrosive primer RB10 Metal Primer (water-based) for galvanised steel and leave to cure for two to four hours before
continuing. To treat red-rusting use an appropriate anti-corrosive primer Metal Etch Primer (solvent-based).
1. Inverted Box Rib (IBR) or corrugated roofing: Apply primer or Promac Roofcote to the prepared surface using a roller (wool or synthetic type), brush or
by airless spraying. Two coats are recommended for optimum durability andquality of finish. Allow the first coat to cure for at least two hours before
applying the final coat.
2. Gutters or down-pipes: Apply primer or Promac Roofcote to the prepared surface using a brush or a small fine foam roller.
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 the coverage rate of the product type (indicated on the packaging).
4. One litre of paint will cover approximately 30 metres of a 30mm-radius down pipe or gutter. This should produce a slight excess of paint, which can be
used to touch-up when required.
Other important information
1. Never apply paint if rain is forecast, or expected, during the day of application.
2. Apply paint after 08h00 (summer) or 10h00 (winter) to ensure that the roof is not too cold or damp.
3. To avoid excessive roof temperatures and to allow the paint to harden sufficiently before any afternoon rain may occur, painting before 12h00 is
4. To reduce colour variations always use the same batch number for touch-up work, which may otherwise be visible on the painted roof.
Description of products mentioned 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. Promac Roofcote 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.