Fineline Product Usage & Roofing Overview 2
Typically in New Zealand, roof and wall cladding is often pre-painted metal substrate, most commonly coated steel.
The steel substrate is metal coated at the steel mill, using a continual hot galvanising process, that deposits a measured mass of zinc/aluminium etc on both sides of the steel strip.
This protective and sacrificial layer will stop red corrosion of the steel substrate, whether or not the product is subsequently painted. The durability performance will vary depending on; the material used, the design of the building, and the environment that it is exposed to.
The use of various proprietory paint systems will enhance the performance of the metal coated substrate, particularly in more extreme environments.
Since Aluminium and galvanised products are difficult to paint onsite, the industry has largely adopted pre-painted metal products, that are painted on a coil coating line. The metal coated steel is pre-treated, then primed, heated and cured, then cooled and topcoated, before being oven cured again, and cooled in a continuous process, without any onsite contamination. The result is generally very consistant, giving predictable and measureable film builds of paint.
This enables the manufacturers to offer warranties consistant with the known durability of the pre-painted metal in any given environment.
The paint systems that are commonly used in NZ are by their nature, "Breathing Membrane Paint Systems". They are not barrier paint systems, like PVC or rubber, that would stop moisture and oxygen at the surface layer. Our most commonly used pre-painted steel products, allow moisture and oxygen to permeate through to the protective galvanic alloy layer of the product. This galvanic coating typically contains a mixture of Zinc and Aluminium.
The job of the zinc is to try to protect the steel substrate by sacrifice, but the Aluminium is present to retain the zinc. It can only do this by forming a protective oxide layer, and to do this it needs moisture and oxygen, for effective self passivation: Hence the paint system must breath.
If the coil coater produced a barrier system, any drilled holes or cut edge, would need to be sealed on site, since any water entering a sealed system would not be able to escape, and would be detrimental , causing corrosion of the steel substrate.
Note: this has been documented in NZ with some now obsolete products.
Author: Philip J. Meyers
27-11-2023