Learn about substrates that are commonly decorated and how the idiosyncrasies of each need to be considered when choosing paints, coatings, stains and finishes.
Timber is a dimensionally unstable material that expands and contracts with changing moisture content. The timber surface is eroded by U.V. light, normally changing to a grey colour leaving cellulose fibres exposed on the surface. Timber also provides a source of nutrient for mould growth. A protective system for timber needs to combat these three sources of aggression; viz. water, U.V. light and mould.
Some timbers contain resins and oils that may affect the performance of paint, such as resin pockets in pine that may soften and bleed through paints, oils in teak that may prevent penetration and adhesion of paint, and anti-oxidants in Kwila, Matai, Spotted Gum and Totara that will inhibit the drying of solventborne paints.
Reconstituted timber products are subject, to a greater or lesser degree, to the same degradation as timber. The majority of these products contain wax to improve water-resistance and wet-strength. These waxes may be dissolved in hydrocarbon solvents present in solventborne paints and then be deposited on the film surface when the solvent evaporates. This wax layer may upset the gloss and finish of the paint system and seriously retard the drying of solventborne paints by blocking the ingress of oxygen needed for curing. Particle boards containing Cedar may cause unacceptable staining.
Sharp edges on timber are very difficult to apply paint to as paint tends to flow away from edges, leaving weak spots. For best results these sharp edges should be sanded to a rounded profile.
Some factory applied primers, due to the demands of the application process, are not able to deliver everything needed in a wood primer. Check the instructions on the back of LOSP timber as it may require sanding and priming with a quality solventborne primer, such as Resene Wood Primer (see Data Sheet D40), before topcoating. Resene supplies a high quality factory primer, Resene True-Prime, to some timber mills, which overcomes these issues and alleviates the need for extra sanding and the extra coat of solventborne primer. If in doubt sand and reprime precoated timbers.
Painting of such treated timber should not commence until all solvent odour has dissipated.
CAUTION: Sanding dust from some hardwoods is considered carcinogenic and all timber sanding dusts must be considered potentially harmful.
Always wear an effective dust mask.
Sanding dust from old lead or chromate based paints or old building materials containing asbestos may be injurious to health if inhaled or ingested. Seek expert advice and read the Resene Putting your safety first brochure if the presence of these materials is suspected.
NB.
Within any species of timber, the durability of any single piece will vary depending on where it is taken from the tree and how it is cut. As a simple rule, quarter sawn timber is much more durable than flat sawn.
With flat sawn weatherboards and planks, the bark-side will tend to split and crack more than the centre-side.
Timbers vary in their ability to resist weathering. This relates to paint performance, particularly over timbers that have been exposed to the weather. Although Resene recommends sanding timber that has been exposed for more than one week, this is somewhat conservative in order to avoid the worst case scenario. There is a simple rule of thumb – the denser the timber, the greater its weather resistance. The one week restriction applies to Cedar, Pine is satisfactory for three weeks and other timbers fit into this scale according to their relative density.
These are purely botanical terms, which in fact bear no relationship to the relative hardness of the timber, for example, Balsawood is strictly speaking a hardwood. Hardwood is the product of broad-leaved trees and contains a complicated and distinctive pattern of vessels or pores, which are much larger in cross section than the surrounding fibres. A good example is Eucalyptus.
Softwood is the timber from conifers of which Pine is the best example. This wood does not have pores but consists of tracheids, which are rather uniform in appearance.
Sapwood is the band of outer layers just under the bark. As the tree grows the sapwood converts to heartwood, which is usually darker in colour. In most trees, especially those with dark heartwood, the sapwood is easily identified by its lighter colour. Due to the presence of nutrient materials in sapwood, such as sugars and starches, it is more prone to attack by decay and fungi. This can be overcome to some extent by treating with a preservative, a process far more difficult to achieve in the heartwood because of the dense cells.
Similar species have different degrees of durability and natural resistance to attack by fungus and insects. Timber may last for thousands of years if appropriate protection is given from the four destroyers of timber; weathering, fire, decay and insects.
All of these reconstituted wood products share the benefit of uniform strength in all directions and the predictability this brings. The requirement to include waxes to improve water resistance may lead to problems of drying with auto oxidatively curing systems. Waterborne systems, moisture-cured coatings and two pack urethanes are therefore recommended.
A timber similar to Western Red Cedar with similar stability, durability and strength. Like Western Red Cedar it has a strong tendency to stain waterborne paint systems.
A softwood with an almost white sapwood and pinkish-brown heartwood. Its most striking feature is the very prominent, evenly spaced growth rings. For this reason it generally does not accept paint very well, although penetrating stains are not considered problematic. It is used for light framing, roof trusses and exposed beams.
A pale greyish to yellow-brown colour with slightly interlocked grain and a moderately coarse texture. Erima is used for mouldings, panelling and skirting and the treated grades are suitable for weatherboards. The timber takes coatings very satisfactorily
A medium density softwood that is established as a commercial alternative for New Zealand Kauri. Displays very similar appearance and working properties.
A dark purplish-red timber that is very dense, hard and durable. The texture is fairly coarse and requires filling for the finest finishing. When used as decking, it does not initially accept penetrating stains due to its density. Acceptance improves on weathering.
Light brown sapwood, reddish brown heartwood with a characteristic 'speckle'. An outstanding all round timber that finishes easily and beautifully.
Kwila is native to New Guinea, Fiji and Queensland. Sapwood is yellow but the heartwood is dark brown. Heartwood is very durable and is favoured for power poles, wharf and bridge construction, boat keels, decking and sleepers. Kwila will accept paint and may be painted with either waterborne or solventborne systems. Its coarse open grain does not lend itself to varnishing unless the grain is filled.
A medium density softwood with an attractive yellow-brown colour. The growth rings are very prominent and it may contain many small tight knots. Used for framing, fencing and occasionally the heartwood is used for feature cladding. The very dense latewood may result in early paint failures and the timber is slightly corrosive to steel.
LOSP is an excellent and convenient method of preserving timber. Its greatest asset is that it can be successfully used on formed and cut timber and joinery ensuring that no timber is exposed that has not had contact with the preservative. Timber delivered to site may contain some residual solvent, which must be allowed to evaporate before painting. Fillet stacking out of direct weather is an appropriate method and the timber should not be coated until it is completely free from solvent odours.
Whitish sapwood, yellowish-brown heartwood with the pleasant characteristic smell of cypresses. The timber has a fine and even texture. A high grade timber, useful for a wide variety of applications including boat building, furniture, framing and cladding. Takes paint coatings reasonably well but the presence of natural resins may lead to patchy acceptance of stains.
Warm orange-brown timber with an attractive interlocking grain. Primarily used for high quality furniture and panelling. Has a coarse texture, which requires filling to produce a smooth finish. Takes standard varnishes well.
Narrow white, non durable sapwood, with a yellow-brown heartwood that darkens to a deep red-brown on exposure to air. A magnificent timber traditionally used for exterior joinery and interior flooring. Takes paint and clear finishes very well, although the natural extractives severely hinder the curing of solventborne finishes. Waterborne systems or moisture-cured coatings and two pack urethanes are therefore recommended.
A mellow golden brown colour with a straight grain. This timber is known for its for strength and durability. Its somewhat coarse texture may require filling to achieve the finest finish. The natural tannins may slow the dry of solventborne finishes and cause corrosion in steel fixings.
An excellent all round timber, which when well cured, produces very few finishing problems. Knots will tend to shrink, loosen and exude resin. Premature coating of some treated timbers before the treatment has become fully fixed may lead to staining problems.
The heartwood of Rewa Rewa is dark red to purple-brown. Timber cut from Rewa Rewa is tough, hardwearing and is easy to work with. The sapwood of the timber is very susceptible to attack by borer. Its most popular use is for craft woodwork.
Reddish brown, streaked heartwood, uniform pale brown sapwood, with a distinct lighter intermediate zone capturing some interesting colour effects. Widely used for framing, cladding and fine furniture. Rimu finishes beautifully with either pigmented or clear finishes.
A useful substitute for Mahogany, has similar colour and grain but a finer texture. Produces a superb finish.
The heartwood of Totara is an even reddish-brown with indistinct growth rings and a very straight grain. Although very durable and easy to work with, it may show brittleness across the grain. As well as playing a central role in Maori culture, Totara was seen as the ideal material for fencing and exterior joinery. The same constituents that naturally preserve the timber also affect the curing of solventborne finishes. Waterborne paints or moisture-cured coatings are therefore recommended.
A dark brown to salmon-pink timber; lightweight, knot-free, stable and durable with a somewhat coarse texture. The surface will erode under natural weathering fairly quickly to leave a stable mat of cellulose fibres on the surface. Cedar is used extensively for exterior joinery, interior cladding and interior panelling. It takes paints and varnishes well but may cause staining of waterborne paints and corrosion of steel nails.
Concrete is the complex reaction product of aggregates, silica sand and calcium silicates present in cement. Lime is produced during the setting reaction that slowly further reacts with the silica sand to form more cement. This lime production accounts for the high initial alkalinity of concrete and is influenced by the amount of water added, type of aggregate and additives used. This high alkalinity may saponify (lime burn) many alkyds and PVAs. It is recommended that concrete is left for 28 days before painting with alkali-resistant paints, such as 100% waterborne finishes, three months for PVAs and one year for solventborne finishes. Excess lime may migrate to the surface as a milky deposit, efflorescence or lime staining. Resene Limelock (see Data Sheet D809) is recommended on all fresh plaster to prevent lime staining. Resene Limelock (see Data Sheet D809) is a preparatory coating designed to cure and seal cementitious surfaces by retaining moisture necessary to achieve cure and trapping free lime, minimising downtime between the completion of plastering and commencement of painting.
The cement matrix is slowly eroded by acids normally present in rain, so that old concrete has a weak, unbound layer of sand on the surface.
The above holds true for all concrete products although the different forms do have some individual differences:
Cement renders (including EIFS, rough cast and stucco): May have weak surface layers due to water loss from relatively thin layers leading to poor curing. Many renders have added lime leading to very high alkalinity.
Concrete masonry: The very low slump mixtures used in this area reduce alkalinity but increase porosity. Mortars used for jointing and reinforcing are generally highly alkaline.
Concrete roofing tiles: The factory applied cementitious coating breaks down on extended weathering to a fine powder which, if untreated, provides a weak substrate for paint.
Fibre reinforced cement: Low incidence of alkalinity and efflorescence in flat sheet; greater danger in moulded sheet. Some possibility of mould release oil contamination.
Glass reinforced cement (GRC): These very dense and glossy surfaces may also be contaminated with form release oils and defoamers.
Old fibre reinforced cement (prior to 1982): May contain asbestos. Contact local Health Authorities for advice on preparation work.
Poured concrete: More likely to have efflorescence and laitance due to higher slump mixes. Strong likelihood of form oils and/or curing agents being present.
Floors are prone to a wide range of contamination, so surface preparation is the critical factor in achieving a long lasting paint system. New concrete floor slabs must be allowed to cure for a minimum of 28 days prior to painting. The preferred surface preparation is by diamond grinding or captive shot blasting. An alternative method is acid etching. Some new floors may have had a curing membrane applied or may contain additives that harden the surface. These treatments result in a surface that is usually resistant to dilute acids and that requires alternative preparation by mechanical abrasion or captive shot blasting. All concrete floors must be checked for excess moisture before coating. In the case of new concrete the test must be carried out after the minimum recommended curing period.
Paper is a mat composed mainly of cellulose fibres derived from either a chemical or mechanical pulping process, may be bleached or unbleached, with or without resin additives for wet-strength, and with or without surface treatments, such as coatings or machine calendaring. Paper for the most part, presents an excellent surface for painting and waterborne paints are excellent in this area. They are normally applied direct to paper. There are however some areas where the use of a solventborne sealer, such as Resene Sureseal (see Data Sheet D42), is advisable.
Yellowing of unbleached paper – unbleached paper in the presence of U.V. light will yellow due to the formation of dye-like substances. This may cause discolouration of the first coat of a waterborne paint applied directly to the paper. Use of an appropriate sealer stops this.
Old wallpaper:
Wallpaper should be checked prior to painting to determine the condition of the coating used. Some old wallpapers used a weak coating, which provides an unsatisfactory base for painting. If the old coating will come off as a powder when scratched with a fingernail, an appropriate sealer will condition the surface successfully.
Wallpaper printed with metallic links should have a test area painted to ensure there is no reaction with paints. Any sign of staining would indicate that sealing is necessary.
Some vinyls contain plasticisers that can migrate into the paint and leave the paint soft and sticky. Test a small area with the topcoat paint, if the area remains tacky after 24 hours seal the wallpaper with Resene Vinyl Wallpaper Sealer (see Data Sheet D406).
Problems may occur due to differences in the porosity of various stopping layers and the paper surface of the paperfaced plasterboard.
For new construction projects ensure that the appropriate level of finish is specified especially if critical lighting conditions exist.
To minimise the occurrence of sheen banding (seeing the position of the jointing compounds after painting) sealing the plaster and board can be done prior to the application of other surfacing products.
Gypsum plaster is the product of mixing gypsum (calcium sulphate hemi-hydrate) with water. The gypsum takes up water and sets forming the di-hydrate. The set plaster is relatively weak compared to cement renders and always requires a penetrating surface conditioner. Gypsum plaster is slowly soluble in water and should not be used in wet areas.
Pure gypsum plaster is chemically inert but impurities, such as sodium sulphate, may cause efflorescence and additives, such as lime putty, may cause the surface to be alkaline. If gypsum is deprived of water before setting, such as thin films over porous surfaces or thin films on a hot day, setting will not take place and a powdery layer will be left on the surface.
Fibrous plaster is cast from moulds normally using a tallow-based release agent; residues of this may cause painting problems. Where tallow residues are suspected, usually on glossy areas, clean the surface with Resene Emulsifiable Solvent Cleaner (see Data Sheet D804).
Some materials used as set retarders may solubilise iron that is present, which may result in a stained appearance after painting.
Special attention must be paid to thin layers of uncured plaster. Either remove and replaster if necessary or reinforce with additional treatments.
All metals corrode when exposed to the atmosphere. Some metals, such as stainless steel and chromium, form an oxide layer that is very stable and protective, while other metals, such as zinc, corrode to produce a weak non-protective layer.
Most metals belong to an electrochemical series and when in contact, one with another, electrochemical cells are set up, which leads to rapid corrosion of the less noble metal. For example, a copper pipe laid over a zinc roof will lead to rapid corrosion of the zinc and indeed water dripping out of a copper overflow and running over zinc will cause problems. For similar reasons lead flashing and lead nailheads are precluded from use on Zincalume. Other problems may occur when a dissimilar metal, such as steel wool, used for surface preparation may leave small fragments embedded in the surface leading to a corrosion cell. Mild steel nails, and metal shavings from drilling or filing, may also cause rapid corrosion on galvanised roofs.
Zinc is also subject to ‘inert catchment’ corrosion. Normal rain is slightly acidic having dissolved quantities of carbon dioxide in it. This is mildly reactive to zinc but because of the small amount involved, is not normally a problem. However, where large amounts of rainwater is collected in one spot, such as where water runs over a painted roof then into a galvanised gutter or an unpainted extension, rapid corrosion can occur at that spot.
Generally the oxidation layer on aluminium is dense, stable and is suitable for painting after careful cleaning to remove other contaminants. Sometimes however, aluminium may be prone to ‘filigree’ corrosion and/or pitting. In these cases the surface must be abraded to bright metal using fine wet and dry paper. Several chemical preparations are available for cleaning aluminium. Should such a product be used it is imperative that all residues are rinsed off before painting. Very highly polished aluminium can be difficult to adhere to and should be wet sanded to a matt surface. Round off any sharp edges on the aluminium profile to allow better film build. Note: Possible corrosion may occur at mitred joints in aluminium where the cut edges are unprimed. This coating system may not provide protection for these areas.
Copper may corrode to form red and black copper oxides and green ‘verdigris’ basic copper carbonate. These corrosion products are relatively stable in themselves but are not a suitable foundation for painting and must be removed. Heavy excess corrosion may be removed by scraping while wet sanding to bright metal finishes. Hydrogen sulphide is particularly aggressive to copper causing rapid corrosion in thermal areas.
Zinc is a reactive material that will corrode to basic zinc carbonate normally and mixed with basic zinc chloride in coastal areas. The form of the corrosion product is not protective. Coating of the metal is recommended as soon as possible and in this case washing with detergent or Resene Emulsifiable Solvent Cleaner (see Data Sheet D804) is all that is necessary. Even though the surface is cleaned initially, it cannot be overstressed that roofs in particular need washing down prior to each day’s painting if there is any danger of windblown salt depositing on the surface. Where corrosion has taken place this must be carefully removed, leaving behind as much of the remaining protective zinc layer as possible. Wet abrasive removal is recommended. Highly polished zinc and galvanised steel may be difficult to adhere to and should be wet sanded to a matt surface.
Lead is a relatively inert material and normally requires only cleaning with detergent or Resene Emulsifiable Solvent Cleaner (see Data Sheet D804) to prepare it for painting. Lead is a toxic metal and attention must be paid to personal hygiene when handling or working with the material.
See more about dealing with lead based paint.
As the name implies this material is generally corrosion free. However some grades in extreme environments may develop some surface rust. This is best removed by abrading with wet and dry sandpaper or for larger jobs by ‘brush’ blasting.
This blend of aluminium and zinc behaves predictably somewhere between the two. Although it is much more corrosion resistant than zinc, it is still recommended that it is painted as soon as possible after careful cleaning with detergent or Resene Emulsifiable Solvent Cleaner (see Data Sheet D804).
Mild steel, for its strength to weight ratio and cheapness, is one of the most widely used construction materials. However, it readily rusts and must be painted to prevent this corrosion and to give it a decorative appearance. Mill scale found on new steel is a hard, brittle coating of several distinct layers of iron oxides formed during processing of steel by hot rolling of girders, tank plates and other structural shapes. Usually bluish black in colour, mill scale cracks and fissures readily and is permeable to both air and moisture. Rusting at the mill scale steel interface occurs and in time the scale sloughs off due to the pressure created by the rust layer. Mill scale is cathodic to the steel substrate and if left in place, corrosion will occur as a result of the electrical potential difference between them.
Rust is a mixture of oxides of iron formed by the action of air and water. It is voluminous and occupies about one and three quarter times the volume of the steel from which it originated. Rust forming under a paint coating or through breaks in the coating may burst through or may creep under the coating resulting in flaking so that repair is both difficult and costly.
It may cost a little more for a well prepared surface, but as the paint coating will last many times longer, the overall cost saving in maintenance will justify the initial expense. Other types of steel, such as low alloy steels like Austen 50, which are selected in areas requiring increased strength, hardness or improved resistance to corrosion, may also be prepared by the following surface preparation.
These materials are inert and porous, but offer an excellent surface for painting. The surface simply needs to be thoroughly clean and dry. Their porous nature may mean that inadvertent stains may be pulled deep into the surface, which may make removal difficult. This may be a particular problem when clear finishes are applied.
These materials are similar to that described immediately above except their surfaces are even smoother. Surface preparation requires careful cleaning followed by a coating with high specific adhesion properties.
These materials, although slightly textured, are non-absorbent. Cleaning with detergent or Resene Emulsifiable Solvent Cleaner (see Data Sheet D804) will prepare the surface satisfactorily for painting. These non-absorbent surfaces will require coating with specific adhesion primers particularly if the surface is to be stressed, such as a floor.
These materials have generally weak surfaces that are not particularly suitable for overcoating. Deeply penetrative primers may be used to consolidate the surface, but such treatments may result in spalling of the entire consolidated surface. Strengthening may be accomplished with Resene Stone Strengthener (see Data Sheet D904), which deposits silica crystals into the matrix of the stone without changing the essential morphology of it. Cleaning should be as gentle as possible in order not to damage the surface. Steam cleaning following fungicidal washing is the preferred method. Penetrating siloxane waterproofing treatments are often the best coating option. For further information refer to the Historic Places Trust for the conservation of historic stone structures.
These materials are generally based on cementitious binders or use a polymer matrix such as epoxy or unsaturated polyester. The former should be prepared as for cementitious and the polymer based materials prepared as for GRP and GRE.
Bitumen substrates start their life as soft solvent sensitive materials but, when exposed to the atmosphere for long periods, oxidise to hard solvent insensitive materials. If painted early the bitumen will remain soft underneath and in some circumstances bitumen may be overcoated with very fast evaporating solventborne paints, such as roadmarking paints. When bitumen is overcoated early in its life with acrylic systems, the bitumen will remain soft for a very long time. EPDM substrates have a high mineral oil content that can pull through solventborne paint systems, affecting the adhesion.
The key aspect in painting baked enamel surfaces is adhesion. These surfaces are difficult to adhere to and require a special adhesion primer. Once primed the materials may generally be treated as normal. Polyethylene and polypropylene however, are exceptionally difficult and even special adhesion promoted primers do not develop what could be categorised as excellent adhesion. Furthermore, these materials soften under the influence of heat and overcoating with dark colours should not be used.
Building plastics, such as UPVC, may be directly overcoated with good quality adhesion promoted waterborne finishes. These thermoplastic materials are sensitive to solvents and may also soften under the influence of heat. Dark colours should not be used to avoid warping.
Canvas may be subject to all weather conditions and is prone to mould growth. Ensure all cloth surfaces, such as exterior umbrellas, are dry before folding or storing away.
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