COP v3.0:maintenance; overpainting

16.8 Overpainting 

To achieve a satisfactory result when overpainting it is vital that preliminary preparation is carried out before painting, and the same cleaning process can be used for new, weathered or prepainted roofs.

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.

Thoroughly clean the surface with fresh flowing water and a medium stiff nylon bristle broom or water blast at 20 MPa ( less than 3 000 p.s.i.). When cleaning the profile of any dirt, lichen or mould, care should be taken not to drive water under laps or flashings.

Rinse the profile swages thoroughly, as any remaining dirt will dry and impair subsequent paint adhesion. Particular attention should be given to the drip edge where the final 15 mm is prone to heavy dirt build-up, and to the coating at the front edge of tile profiles as dirt collects in this area and gives rise to mould and algae growth.

Fallout around flues needs to removed before painting.

Allow the roof to dry.

16.8.1 Unpainted Cladding 

The practice of leaving metallically coated roof cladding to weather before painting is no longer recommended. With the latest developments in primers, roofs can now be painted immediately after installation, and it should be done within one month.

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.

After the primer, two coats of acrylic roofing paint should be applied in the selected colour to provide a paint thickness of 50 µm (typically two brush applied coats achieve 50 µm ).

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.

 

16.8.2 Repainting Painted Claddings 

Repainting painted claddings should be regarded as part of the maintenance programme to extend the life of all metal claddings.
There is an optimum time at which to repaint claddings, and this decision should be made by the owner after specialist consultation.

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.

 

16.8.3 Weathered With White Corrosion 

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.

 

16.8.4 Red Corrosion 

Red corrosion, or corrosion at the edges of the profiled steel cladding should be prepared as follows:

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 .

Coat the whole prepared areas with a water based galvanised iron primer and apply two topcoats of acrylic roofing paint to the dry, primed area.

16.8.5 Paint 

High quality, 100% acrylic paint can give a service life of up to 10 years when applied to specification on correctly prepared metal surfaces. This lifespan will vary slightly with colour, roof orientation and the aesthetic requirements of the situation, but poor paint curing will downgrade durability.

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.

 

16.8.6 Brush Application 

The use of nylon brushes is advisable, and these should be wetted with water prior to use to avoid clogging, and in warm weather should be washed out completely at every rest break.

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.

 

16.8.7 Roller Application 

Apply to the spreading rate and film build specified for brush application. Dampen the roller prior to use, then load up the roller and apply two full even coats, allowing sufficient drying time between coats.
Apply with even pressure, and do not over-roll. Profiled rollers are available for corrugated profiles, but some profiles may require the combined use of brush and roller of the paint for complete application.

 

16.8.8 Spray Application 

Apply to the dry film building specification of a minimum of 50 µm. Airless spray equipment can achieve very high film builds giving rise to runs and uneven coatings. When using spray equipment on hot or windy days, the paint spray may dry before it has reached the metal, which leaves a sandy appearance and feel. This is not aesthetically acceptable and will not provide the even coating required, even though it may measure 50 µm.

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.