Code of Practice v3.0 Online
The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
The surface preparation, painting and over-painting of metal roof and wall cladding should be carried out by specialist contractors.
To achieve satisfactory adhesion and optimum results, all dirt should be washed away and cleaning agents such as soap or detergent should be fully washed off the surface prior to paint application. Over-painting a dirty or wet surface results in poor adhesion of the paint and consequently a reduced life; it can also cause premature corrosion of steel cladding.
For new AZ coated cladding, only acrylic galvanised iron primers should be used, as solvent-based primers may damage the coating. A solvent-borne corrosion resistant galvanised-iron primer should be used for optimum performance on weathered zinc coated cladding over twelve months old.
If the cladding is weathered but shows signs of white or red corrosion or damage back to the metal base, proceed with painting as described 16.8.3 Weathered With White Corrosion and 16.8.4 Red Corrosion.
It is also known as 'the life to first maintenance, ' and is the time before excessive chalking has taken place or the top coat weathered away.
Painted products can be readily over-painted with high-quality acrylic roof paints to extend the life of the roof cladding and if over-painting is carried out while the top coat is still in sound condition there is no need to use a primer.
To prepare the surface for painting all pre-painted products should have their surfaces abraded with a fine grit sandpaper, stiff nylon bristle broom or similar, to improve the adhesion. Care should be exercised not to sand through the existing paint surface on external bends.
If the white corrosion can be easily removed and no red rusting has developed, the roof cladding should be cleaned as outlined above. Neutralise the areas where white corrosion is present with a proprietary metal cleaner designed for this purpose and follow instructions on the container closely. If all residues from the metal cleaner are not removed before painting, poor paint adhesion will result.
Painting should take place as soon as possible after this pre-treatment. The presence of white corrosion indicates that the primer has been consumed and so isolated areas of white corrosion should be spot primed once these areas have been cleaned and neutralised. If all external bends are showing corrosion, coat the whole area with an acrylic galvanised iron primer.
For unpainted products, it is necessary to use a primer over the whole surface.
Manually de-scale and remove all white and red rust by abrading to bright, firm metal, ensuring that the surface is as smooth as possible. Thoroughly clean the roof as described previously and neutralise the corrosion reaction with a commercially available metal cleaner made for this purpose.
This type of solution should not be allowed to dry on the surface before fully washing off, and all residues of the metal treatment should be removed prior to painting . Dry all surfaces before spot priming the cleaned bare red corrosion affected areas with a zinc-rich primer .
Paint should not be applied on wet days, when condensation has not completely dried, on cold days below 10° or hot days above 30°. Windy days are also not suitable, as curing is impaired.
The painting of very hot roofs will result in the evaporation of the solvent before full film formation can take place, and so will reduce the life of the coating.
Ridge ladders should be fitted with protective buffers or rubber pads as they can cause extensive abrasion damage to pre-coated metal cladding. Where it is possible the painter should walk in the pan, but when it is necessary to step on the rib, attention should be paid to sheet overlaps as these may spring up after a painter's weight is removed and reveal an unpainted line.
Decking profiles have a tight roll-formed bend at the top of the upstand and care is necessary to ensure the specified film build is applied in this area.
Apply two full even coats of acrylic paint to a dry film build of 50 µm, allowing sufficient drying time between coats, but do not over-brush and thin only sparingly.
Do not spray before 10.00 am or after 3.00 pm as condensation can be present. Outside this time space steel will also cool rapidly, which will impair curing and can result in incorrect film formation. It can lead to early failure of the coating. These hours could be extended in mid-summer to before 9.00 am or after 4.00 pm.
The gloss and weathering characteristics of oven cured and air dried paints are different, and over time a significant difference in colour may become apparent. Variations in natural light conditions will emphasise these differences producing unacceptable aesthetic variations. For this reason, the whole roof area should be painted and not patch painted and this also is the reason why 'touch-up' paint should not be used.
Spray cans should not be used for repairing scratches on pre-painted cladding. Minor surface scratches are best left because they become less noticeable as the coating weathers. If the scratch cannot be seen from the ground, it should be left alone.