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.
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.