As with all surfaces, the overpaint performance on profiled metal surfaces is dependent on correct surface preparation as well as application. This clause provides generic recommendations; in all cases, the advice of the paint supplier should be studied and followed.
Thoroughly clean the surface with fresh flowing water and a medium stiff nylon bristle broom, or water blast at 20 MPa. When cleaning the profile to remove dirt, lichen, mould, or algae, care should be taken not to drive water under laps or flashings. Detergent or proprietary roof wash may be added (if safe) to aid dirt removal, but if used should be flushed off with fresh water before drying.
Rinse the profile thoroughly, as any remaining dirt will 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 leading to mould, algae, and lichen growth.
Fallout around flues and vents needs to be removed before painting.
Manually abrade the red rust down to bright metal, being careful not to overly remove adjacent metallic coatings. Prime with a metal-rich primer and overcoat with an anti-corrosive primer and compatible topcoat.
The use of rust neutralisers is an alternative but must be used with caution. Many are aggressive to remaining metallic coatings, some retain corrosion salts which may cause undercut corrosion in the future, and some, if not completely removed after application, will accelerate corrosion. If considering this path, use it strictly according to the manufacturer’s recommendations and requirements.
High-quality, 100% acrylic paint can give a service life of up to 10 years or more when applied to specification on correctly prepared metal surfaces. This lifespan will vary with colour, roof orientation and the aesthetic requirements of the situation. Poor paint curing will downgrade durability.
For many paints, a 50µ dry film build is required to achieve optimal thickness, and typically this requires two layers of top-coat.
Paint should not be applied on wet days when condensation has not completely dried, or when ambient temperatures are expected to reach less than 10° or more than 30° within the drying period. Windy days are also not recommended, as curing is impaired.
Ridge ladders should be fitted with protective buffers or rubber pads as they can cause 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, (only at the purlin line) 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.
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, or wrapped in cling film and refrigerated.
Roller application is not preferred for profiled metal roofing as it tends to apply an uneven paint film thickness. The combined use of brush and roller can give good results with tray profiles.
Dampen the roller prior to use, apply with even pressure, and do not over-roll.
Spray application in the hands of a skilled applicator can achieve the most consistent film builds. However, airless spray equipment can produce 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 or durability required.
The practice of leaving metal-coated roof cladding to weather before painting is no longer required. With the latest developments in primers, roofs can now be painted immediately after installation.
For un-weathered AZ-coated cladding, only acrylic galvanised iron primers should be used, as solvent-borne primers may damage the coating. A solvent-borne corrosion-resistant galvanised-iron primer should, however, be used for optimum performance on weathered AZ or galvanised-coated cladding. Wash and allow to dry (see preparation above.
Apply two coats of acrylic roofing paint in the selected colour to provide the required paint thickness. (Typically two brush-applied coats achieve 50 µm).
Repainting painted cladding should be regarded as part of the maintenance programme to extend the life of all metal cladding.
The optimal time to overpaint weathered pre-painted cladding is when the colour has faded excessively, or the topcoat has eroded to the point that the primer is becoming exposed.
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.
If the over-painting is carried out while the topcoat is still in sound condition, there is no need to use a primer.
Apply two coats of acrylic roofing paint in the selected colour to provide the paint thickness. (Typically two brush-applied coats achieve 50 µm ).
It is not easy to obtain adhesion to unweathered pre-painted metal cladding. To prepare such surfaces for painting, abrade all surfaces with 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 purpose of overpainting new cladding is to obtain a colour finish that is not available in pre-painted material, it is often more economical to order pre-primed material rather than try to overpaint an un-weathered pre-painted surface.