The prime ingredients in successfully finishing a construction project are good-quality coatings combined with the correct preparation and application techniques
In our previous issue, we looked at paint systems, design considerations, the options of painting or staining timber, painting concrete, painting masonry and painting steelwork. This issue, we are looking at clear finishes and stains, powder coatings, electroplating, some specialist coatings, and we consider sustainability.
Clear finishes and stains
Clear finishes and wood stains are used on timber where its natural colour is to be retained but where a degree of weather protection is required. They will not stop the timber weathering (colour change) but will slow it down – the rate depending on the transparency or amount of UV-blocking pigment contained in the finish.
Generally, the more pigment in the formula, the less the transparency and the greater the protection given (and less of the original timber colour visible). These coatings provide significantly less protection to the timber than paint and they are often a disappointment because of their limited durability.
In addition to oils, there is a range of finish options.
Film-forming transparent finishes
Sometimes called varnishes, these were oil-based and resin-modified in the past, but are now either waterborne or solvent-borne and the resins on which they are based include acrylics, alkyds and urethanes.
Acrylics have a number of favourable properties, such as flexibility, easy clean-up and UV resistance. Although acrylic film has good UV-resistant properties, it does allow some UV light through to the timber surface. This has resulted in earlier than expected breakdown of these systems.
As developments in UV absorbers, stabilisers and wood-surface modifiers continue to progress, improvements in the performance of these products may occur.
Penetrating or low-build (water-repellent) wood finishes
These are usually solvent-borne and are frequently described as water repellents. These finishes form only a very thin film on the surface of the timber. They have their uses in situations protected from direct weathering, but in most exposed areas can only be regarded as temporary wood-protecting agents. Maintenance of such coatings could be required at least six-monthly.
The use of vegetable oils (for example, linseed oil) for finishing timbers to be used outdoors is not recommended unless the oil has been modified by the addition of suitable fungicides. Oil will act as a food source for mildew and may cause rapid discolouration (blackening) of the timber surface.
Film-forming stains
Sometimes described as solid colour stains, these are generally waterborne (acrylic), are more opaque than the solvent varieties and behave like dilute paints in terms of durability. They are not recommended for high-wear areas. Waterborne stains are generally fast-drying and are more likely to show lap marks than solvent-borne stains.
Penetrating semi-transparent stains
These are designed to show the texture and grain pattern of the wood, while still protecting the surface from the weather. These are low-build stains containing a small quantity of pigment.
Specialist clear finishes and stains
A variety of specialist clear finishes and stains are also available to provide colour and/or protection to surfaces other than timber. These coatings include:
Clear glazes for concrete surfaces (use with care on horizontal surfaces where slip resistance is required).
Colour-imparting stains for use on concrete surfaces.
Sealers for stone, concrete and tile.
Lacquers for polished metals such as brass or copper.
Powder coating
Powder coating protects like paint but is much tougher and more durable. The 100% solid powders release no solvents into the atmosphere and are considered nearly pollution-free during application.
The powder systems most commonly used for external use are thermosetting polyesters (because of their weathering and chemical resistance). These can be applied to a wide range of substrates including aluminium, brass, steel and zinc alloys.
Other options for external use:
High-performance polyesters designed to retain their original colour and gloss better than standard polyester. These powders are not available in bright colours such as reds, yellows, oranges or pinks.
Fluoropolymers, which are purpose made-to-order powders, available in lower gloss levels and designed to be the most durable powder coating system for external use.
Epoxies, which are slightly harder and more abrasion resistant than polyesters but not as resistant to ultraviolet light (they tend to chalk). They are more suited for indoor use unless appearance is unimportant and/or excellent chemical resistance is required.
Powders based on polyurethanes, acrylics and blends of these.
Thermosetting powders are usually applied by an electrostatic spray. Powder particles are electrically charged by the spray gun, while the item being coated is earthed to attract the charged particles to give a relatively uniform thickness across exposed surfaces. The item is then heated under controlled conditions to make the powder melt and flow into an even film. Powder coatings must be applied to a correctly prepared and clean substrate by specialists.
Powder-coating thickness must be between 40 - 125 microns (1000 microns = 1 mm).
Films less than 40 microns thick are undesirable. They may be prone to pinhole penetration to the metal surface and may erode too quickly in terms of acceptable product life. Excessively thick films may be unacceptable in appearance, because of surface roughness.
A sharp radius on any extrusion edge should be avoided. The minimum recommended radius is 0.7 mm.
A sharper edge can lead to thinning of paint cover and premature coating failure.
Metal coatings – electroplating
Electroplating is an electrochemical process where a coating of metal is deposited onto the article. This is done by passing an electric current through the body of the item to be plated while it is immersed in a chemical solution containing the plating metal.
Most metal substrates will accept electroplating, although differing types of chemical pretreatment are necessary for some combinations. The most usual body metals are brass and steel.
Platings that may be suitable for external use (check with the supplier first) are:
Hard chrome plate.
Copper plate.
Nickel.
Zinc nickel – provides more than three times the corrosion resistance of zinc plating.
Gold.
Silver.
Specialist coatings
There is also a wide range of special-purpose coatings such as:
Zinc-rich paint, for protecting prepared steel surfaces from corrosion.
Epoxies, to provide a very hardwearing abrasion-resistant surface over a range of gloss levels.
Aluminium paint, to seal resinous timbers.
Zinc epoxy primers, for steel protection in chemical and industrial plants.
Chlorinated rubbers, for steel in marine environments.
Urethanes, where resistance to abrasion, moisture, alkalis and acids is required.
Etching primers, to improve the bond of finishing coats to a metal surface.
Bitumen emulsions, for low-cost anti-corrosion and protection properties.
Abrasion-resistant polyurethane paving paints.
Paints described as ‘cool colour’ possess unique formulations and materials that allow them to absorb light (in the visible spectrum), so that they appear the same as conventional paints, but allow them also to reflect most of the heat energy (the infrared part of the spectrum) falling on the surface. The result is that surfaces remain significantly cooler when darker colours are used.
A note on sustainability
Coatings are manufactured using a wide range of organic and inorganic materials. They can contain components that can adversely affect the environment at different stages of the product’s life cycle. Solvents and toxic substances can be released during manufacture, application, throughout the service life of the coating and during the process of its disposal.
When paints with volatile organic compounds (VOCs) dry, potentially toxic solvents evaporate into the atmosphere over a period of time, posing health risks in internal spaces and contributing to environmental pollution.
Environmentally friendly waterborne paints, on the other hand, reduce the level of toxic solvents and emit mainly harmless water vapour into the air. Paint finishes that include natural formulations with water-based solvents, vegetable-based binders and naturally occurring minerals for pigments have been developed to replace solvent-based paints for many common applications.
Low-VOC paints, sealants and adhesives are almost odourless, safer to apply, easier to clean up and less toxic to dispose of.
Some larger paint manufacturing companies offer paint recycling schemes.
Strategies for design and construction:
Use safe formula paints and stains – select finishes with no VOCs or low VOCs.
Make use of material safety data sheets, which contain information about environmental risk and identify personal health hazards.
Avoid products that warn of neurotoxin effects or chemicals that may cause cancer, birth defects or reproductive problems.
Select products certified by a reputable agency such as Environmental Choice New Zealand.
Select the right product for the particular application.
Select a product that will last for the longest possible time rather than choosing a cheaper inferior product that will need more maintenance and more frequent repainting.
This article was first published in BRANZ’s Landscape Construction book, Second Edition and is reproduced with permission.
The book covers the design, building and planning rules as they apply to landscape construction and provides guidance on design and construction to ensure good practices are followed. It can be purchased at www.branz.co.nz.