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Ask an Expert

Preservative treatments & finishes

Like to know what wood to use building a pergola or framing a barn? How about the difference between tung oil and polyurethane as a floor finish? How to revive decking or deal with merbau stains? Or how to meet the building code or bushfire standards? Or ask about the environmental advantages of wood or forest certification?

Q. I have just had a jarrah deck laid on a Victorian Cottage. How long should I wait before applying a sealer? Does the timber need to be cleaned first? What are the advantages and disadvantages of stains vs oils and water based products vs oil based products? Is it best to use a tinted product/stain or clear sealer on Jarrah?

Some say decking should be weathered for a time so the coating can penetrate better. This might be based on the idea that freshly planed timber has a glazed surface (mill glaze or planer's glaze) that prevents absorption of the oil. Research bodies have been unable to duplicate mill glaze in the laboratory, but you will find more information about it if you write the words "mill glaze" in your browser. Others say timber should be coated as soon as possible, before UV affects the surface and the natural colour starts to weather off to grey. We are inclined to the latter view, ie. coat it straight away. If it's a hardwood that is likely to leach tannin, such as jarrah, a preliminary scrub with a deck cleaning product is worthwhile before you apply the oil. This is not something we've tried ourselves, but it's recommended by the finishing companies to remove excess tannin. Apart from that, we don't think there's any need to delay coating your deck. Clear oil will look great on jarrah but needs regular maintenance to keep the colour. Using a pigmented stain will achieve slightly better UV resistance, and therefore a slightly longer life between coats - however, the density of the wood means that neither type of coating will penetrate much below the surface.

Q. I'm not sure this is the right forum for my query but I hope you can help. My self and my husband bought a table made of recycled Oregon in Sydney when we lived there. It's 15 years old now and needs to be re-varnished. I need to know how to go about it and whether it needs special treatment. I now live in Ireland.

Our answer depends on the condition of the table. If the varnish is in good condition but just looks a bit dull, you only need to sand it lightly and re-coat. If the varnish has worn through, or is peeling, you will need to sand the tabletop more thoroughly, making sure you remove any perished or loose varnish. If the wood is marked or dented, it won't be practical to attempt to sand it by hand. In this case the best option would be to remove the tabletop and have it sanded by a joinery company with a wide-belt sander, so as to get an even surface. If you are not sure wherther the existing varnish is water-based or solvent-based we suggest you seek advice from a varnish manufacturer regarding compatibility.

Q. Over time we have ripped up the carpet in our 1950's home to expose beautiful cypress pine floorboards which have been left untreated. We like the rustic mat look. In the near future we would like to apply a finish to protect the floorboards from pet stains etc. What finish would you suggest? Could we just use an oil without sanding? Years down the track (10 years time) if the floorboards were oiled can we sand and apply a waterbased protective coat?

Mats are a good idea in high traffic areas, but we would still be cautious about using an oil finish, particularly if there is a possibility of pet stains. Oil finishes are easy to apply but give a dull finish and are not highly stain-resistant. Polyurethane is more practical in our opinion, since it can be wiped clean more easily. Also an oil finish will tend to repel any waterbased coating that you intend to apply later - sanding will remove most of it, but it will still get down into the tongue and groove joint and possibly prevent adhesion of the new coating.

Q. I have a camphorwood chest that I want to paint, is this ok, and how do I go about this ?

Camphorwood chests are more commonly varnished to show the natural colour of the wood, and painting it might affect its value if you want to sell it later. However, if you particularly want to paint it there shouldn't be any difficulty. Just make sure there is no existing coating that might affect the adhesion of the paint, such as oil or wax.

Q. We are having a new blackbutt staircase installed inside our house but the stair people do not include staining/sealing. What product should we use? Does this need to be professionally applied?

We assume there will not be a carpet runner up the centre of the stairs and the treads will be fully exposed to wear. In that case we recommend a floor varnish. There are many suitable products on the market which will give satisfactory service as long as they are manufactured specifically for floors. Traditionally, solvent-based polyurethanes have been considered the hardest wearing products, but today's water based polyurethanes wear well and you are not exposed to a strong solvent odour during application. Floor varnishes can be brushed on and it's not necessary to have them professionally applied. We don't recommend staining stairs - a clear varnish will retain the natural colour of the wood and is more easily touched up in the event that the coating wears through.

Q. Last Year I got an Ironbark deck laid and I then applied Berger Premium oil water based oil in Natural colour. Now it shows a lot of dirt and all of the scratches from the outdoor chairs being moved on it. It looks very flat and terrible. I stained it in December 2012 and am now wondering if I can stain it with something that is oil based to give it that wet shiny look. Would it have to be sanded? Please help I have no idea what to do, it cost alot of money and I feel it was a waste of time and money. Regards, Vicky

You don't need to sand the deck, but it might be a good idea to give it a scrub with deck cleaner to get back to square one. There are several suitable products on the market and you can find them by writing "deck cleaner" in your browser. If the deck is fully exposed to the weather clear oil will need quite frequent application to keep up the natural look. It's hard to put a figure on it, but when it looks as if it needs doing, then it needs doing - at least annually. We weren't sure what kind of chairs you have but it might be worth looking for chairs that are less inclined to scratch the deck.

Q. We are installing a celery top pine deck, and want to know the best finish for this. Appreciate your help please.

Celery-top pine has good weathering qualities and could be left uncoated, in which case it will slowly turn a silvery grey colour under full weather exposure. If you prefer to keep some colour, the usual range of decking stains and oils can be used. Bear in mind that generally the more pigment in the stain (ie. the deeper the colour), the longer it will last between coats. This is because the pigment helps to counteract UV light. Clear oils need the most work - it's hard to put a figure on it, but they need to be applied at least annually and maybe more often depending on exposure.

Q. Would you know if there are any companies offering Thermowood or its alternatives in Australia? If not, what are the reasons, in your opinion, that prevent its distribution over here? As far as I know it enjoys a wide use in many European countries, and can replace CCA treatment in some cases.

Stora Enso Timber produces ThermoWood in Europe so we suggest you inquire through their Australian office in Melbourne, phone (03) 8369 8999. There are no barriers against its use in Australia as far as we are aware.

Q. I am new to the drafting industry and am in the process of drafting a deck/pergola. I have been told by council since my deck is 850mm above ground level it needs to be braced and since cross bracing will be almost ineffective on a height of 850mm I need to concrete my posts in rather than using post stirrups. They recommended H5 treated timber. I am wondering what is H5 treated timber, is it just CCA pine or treated with another product? Is there another product I can concrete into the ground which isn't CCA? Any help or advise here would be greatly appreciated Thanks Craig

You might want to run it past your engineer to see whether there is some acceptable alternative to concreting the timber into the ground. If not, H5 is simply the Hazard Level, as required for house stumps and other items that are exposed to high hazards and/or required to have a long life. Since H5 is not commonly stocked you might put it to Council that H4 is more readily available and according to AS 1684.2 "Residential Timber-Framed Construction" is suitable for "pergolas (in ground) and landscaping timbers". CCA is commonly used for treated pine, but other treatments such as ACQ are available for Hazard Classes up to H5, and may be available in your area.

Q. We are a structural engineering consultancy and we are managing a remedial project in Neutral Bay. The project involves the refurb of an existing outdoor pool and its surrounding timber deck. The pool is located in the harbour of Neutral Bay. I have a technical query regarding the use of treated pine for decking timber in a marine environment (joists and decking boards). The contractor has used H3 treated pine to replace timber joists in some areas. We see a potential durability issue with this as the deck is located over sea water in a harbour and is potentially in the splash zone of tidal action. It is raised and should not become submerged. AS1684.2 provides advice relating to treatment levels and H6 is recommended for marine environments but it seems that this may be excessive as the deck will not be submerged. Can you please provide some advice on recommended treatment levels for pine in this instance? What would the recommended treatment level be? What are the implications of allowing the contractor to use H3 treated pine as this timber is already on site? Could we expect to get 20 year design life out of the treated pine timber?

Hazard Class H6 is actually for timber standing in sea water and the reason for the high level of treatment is not so much for wood rot but rather as protection against marine borers. So it would be an excessive treatment for timber suspended over sea water, even if it is occasionally splashed. On the other hand, H3 treatment is for timber suspended over dry land, ie. exposed to the weather but not necessarily over water. It's hard to say whether H3 will give you a 20-year design life. It might, since salt water is not particularly hazardous for wood - in fact salt is a mild anti-fungal agent. Maybe the life-span of the previous deck will give you some guidance. But if the timber is relatively close to the water, the timber might stay damp for long periods due to moisture evaporating from the sea, which of course is fresh water. So we would prefer to see H4 treatment used if that is a practical option. Unfortunately decking timber is not normally treated to H4 level, so custom treating is likely to be required.


Did you know?

In 2008 seventy percent of known old-growth trees are in nature conservation reserves.