Fully Covered – Applied Finishes (Part 1)

The prime ingredients in successfully finishing a construction project are good-quality coatings combined with the correct preparation and application techniques

Finishes in a range of formulations, including stains, oils, paints and lacquers, are available for application to almost any surface material.

Options include:

• Stains formulated for timber or concrete.

• Clear finishes for timber, concrete, tiles, stone and metals such as brass.

• Powder coating for metals (aluminium and steel).

• Metal coatings.

Paints

• Paints are usually one of two types:

• Oil-borne or solvent-borne paints use VOCs, typically white spirits, mineral turpentine, toluene or xylene, to dissolve and carry the binder, plus pigments and other components. Solvent-borne paints are usually recommended where a hardwearing, impact-resistant finish is required. Paint descriptions include oil, alkyd, oil/alkyd, alkyd enamel and stain.

• Waterborne paints use water as the solvent to dissolve and carry the binder, pigments and other components. Paint types include acrylic, PVA, latex, acrylic/PVA, vinyl, stain and acrylic enamel. Waterborne paints are quick- drying, less brittle with age and can be more durable than solvent-borne coatings.

Paint systems

A paint system typically consists of:

• A primer or sealer formulated for the material being painted (for example, timber or metal) to provide good adhesion for subsequent coatings.

• Speciality primers designed to perform specific functions, such as:

  • Restricting corrosion of metal substrates (for example, zinc-rich or anti-corrosive metal primers).

  • Restricting the bleeding of soluble material from the timber substrate.

  • Etching a smooth surface to provide good adhesion for subsequent coats.

  • An undercoat (optional with many systems) to improve film build and allow sanding of the substrate.

  • Two finish coats – for some materials such as fibre-cement and concrete, some finish coats can be directly applied to the material.

The following sections give basic information on the coating materials most commonly used in landscape construction. More detailed information on coatings is given in BRANZ Good Practice Guide Exterior Coating.

Design considerations

Key design considerations for applying a coating:

• Always use good-quality paint – cheaper paints are generally made to a price and may not perform as well. Coatings should be manufactured to a recognised standard or quality.

• Darker colours increase the temperature of and thermal movement in a material. In timber, this may also increase the risk of the timber splitting and premature failure of the coating.

• Fading and chalking (powdery appearance on the surface) will be more noticeable with a darker colour.

• All coatings will require regular recoating.

• Gloss finishes allow contaminants to be washed off more easily. They are also more able to accommodate frequent cleaning.

• The more gloss a finish has, the more it will highlight any imperfections in the surface – low-gloss finishes tend to mask imperfections.

• A coating system (preparatory and final coats) should be sourced from one manufacturer where possible.

• A solvent-borne coating should not usually be applied over a waterborne coating unless the latter is specifically designed for use under subsequent solvent-borne coatings, such as a rapid-drying undercoat.

Painting or staining timber

Good paint and film-forming stain preparation requirements:

• Remove all weathered timber – paint will not adhere well to weathered timber.

• Remove sharp edges by rounding to a 3 mm or larger radius.

• Sand surfaces. Do not use steel wool or wire brushes, because small iron particles left behind can react with chemicals in the timber or ingredients in the coating and cause unsightly discolouration.

• Remove resin, surface dirt, dust, loose fibres and mildew growth.

Important requirements when applying any surface coating to timber:

• Seal the end grain. The durability of applied finishes is improved when the end grain of the timber is sealed to prevent moisture entry.

• Only coat dry timber to prevent blistering as water dries from wet timber.

• Punch nail heads below the surface of the timber before applying the priming coat. Fill all holes using linseed oil putty or exterior-grade acrylic stopping compound after priming.

• Pre-prime or pre-seal all timber before erection where possible – ensuring all faces and edges of the timber are coated will reduce moisture absorption and enhance coating life.

• Use a low-build penetrating stain for rough-sawn timber.

• Use pigmented finishes for better durability.

• Paint finishes perform better on dressed timber.

• Do not expose primed surfaces for longer than six weeks before painting.

Painting concrete

Concrete surfaces (and clay bricks) accept paint finishes. Most paint manufacturers produce paints with 100% acrylic formulations for use on cement-based materials. While alkyd paints are not generally recommended for use over concrete, they may be used provided the concrete has been allowed to dry or cure for at least three months.

Factors that can affect the quality and durability of a paint finish on concrete:

• Dampness in the concrete. Paint will not adhere well to concrete that is too wet or green.

• The alkalinity of the concrete. It is preferable to leave new concrete or plaster surfaces to weather before painting, so that some of the alkaline salts can dry and dilute. Minimum weathering periods are:

  • For 100% acrylic paints – two months.

  • For alkali-resistant alkyd or solvent-borne paints – four months plus one month for every 25mm increase in thickness over 100mm.

• The surface quality of the concrete – concrete that is being painted should have an off-form fair-faced finish.

• Surface contaminants, which can impair the bond between the paint and the concrete. These include efflorescence, laitance (surface crumbling or degradation), form or release oils, curing agents, dirt, mould and mildew. Remove form oils or curing compounds by thoroughly cleaning, using a stiff bristled brush with a proprietary degreasing compound. For seriously degraded or dirty surfaces, abrasive blasting or power cleaning, with wire brushes may be used.

• Moisture getting in behind the paint system after it has been applied.

Painting concrete masonry

Concrete masonry surfaces can be painted, but to ensure optimum paint performance:

• Remove all lichen, dust and dirt – use water blasting if dirty.

• Ensure the concrete masonry is sufficiently dry.

• Seal porous blocks with a specialised sealer such as a penetrating alkali-resistant solventborne sealer – sealers should be brush or roller applied to ensure that the surface is fully wetted.

Painting structural steelwork

Mild steel rusts easily, so it must be coated to protect against corrosion by hot-dip galvanising and/or painting, to ensure it retains its strength and appearance. Areas requiring particular attention to coating quality are:

• Steel exposed to the atmosphere but not subject to regular cleaning or rain washing, such as where it is protected by a roof overhang.

• Steel enclosed in a structure (water vapour carried in air will cause corrosion of poorly coated steel) where it cannot be regularly checked or maintained.

• Steelwork embedded in concrete – the steel will corrode around the point where it enters the concrete unless it is well protected. If rust does occur. it will cause the concrete to eventually spall.

Steel sections are commonly supplied to a site preprimed but may still require degreasing before further coatings are applied. All damage to the coating (from welding on site, handling and so on) should be cleaned down to the base metal by grinding or by hand tools (wire brushing and/or sanding) and reprimed with the original primer within 1 hour of preparation (to prevent new rust forming on the prepared surface as a result of atmospheric moisture).

For galvanised steel, once cleaned down and spot primed in damaged areas with a zinc-rich primer, coat with a primer designed for use over galvanised surfaces.

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.