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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 realised you guys have replied to my email a few weeks ago. Thank you so much, I have only just seen your reply. I am still stressing over putting my turtles in the pond because of the merbau bleeding and only came across your reply by accident. I guess the next question is am I better to let it bleed or should I stain it. Thank you so much for finding the thread of tannins, it has really put my mind at ease. We have been to bunnings and asked them about stains and they really don't know. They have said we could clean it with the merbau cleaner they supply which will remove more of the bleeding but I am concerned in treating the merbau and that poisoning the water. But in order for the merbau to withstand water from the pond it should probably be treated. I am so worried and am really getting exhausted trying to find the answers.

Most deck cleaning products of the type that remove tannin have an oxalic acid base. They remove tannin but we definitely wouldn't recommend using deck cleaner in your situation as it will contaminate the water. It would be better to let the tannin wash out with the action of the rain. It's not necessary to stain the merbau if you don't mind it turning grey. Merbau is a durable timber and will give good service without a coating. If you particularly want to stain it, a water-based stain would be advisable although we are not sure what effect that might have on the turtles when the rain washes over it and ends up in the pond. So all things considered, it might be best to just leave the wood bare.

Q. I have got a large Chinese carved chest, and in the process of sanding off a dark stain, varnish, as I want it lighter in colour, what would you recommend I do with it to preserve and finish the chest once I have removed the dark colouring?

A nice natural finish for furniture is Scandinavian Finishing Oil. Several companies manufacture suitable products, eg. Solver Paints. You can find out more from this link: If it happens to be a camphor wood chest you should not seal the interior as that would seal off the camphor aroma which acts as a natural moth repellent.

Q. I would like to know if you recommend polyurethane finish for domestic timber flooring, or is it better to use finishes such as tung oil.

Polyurethane is an excellent floor finish, providing a hard wearing surface. There have been some issues with solvent-based polyurethanes which act as a very effective glue when they penetrate into the tongue and groove joint. If that happens, they stick the edges of the boards together and during hot dry spells the boards may split since they are unable to move at each joint as they are meant to. You can find more information on the net if you write "edge bonding floorboards" in your browser. So solvent-based polyurethanes should be applied with care - not by flood-coating and never thinned. Water-based polyurethanes are less likely to cause edge bonding since they are a little more flexible. Tung oil is also a satisfactory floor finish but is less hardwearing than polyurethane and it is advisable to place rugs in high traffic areas. Some manufacturers recommend maintenance with a floor polish to improve wearability. However, tung oil does not cause edge bonding.

Q. Any recommended stains for the Spotted Gum? The feature is on the south side of the building. It won't be painted but left natural.

We try to avoid quoting brand-name finishes (a) because we haven't tested any, and (b) if we omit to mention some the manufacturers get upset. Instead we talk about generic types of finishes. For more information on this subject, go to the Wood Solutions website via this link: Guide no. 13 is titled "Finishing Timber Externally" and it can be downloaded free of charge.

Q. I designed a home 15 yrs ago with a post and beam timber frame, 300 x 300 posts, 500 x 120 beams. I've found a small amount of rot in the back of one of the beams. It seems water ingress over the past 15 yrs has caused this. It only became obvious when we were working on the ceiling and removed a large section of the ceiling (pb). My question is 'assuming we can stop the water ingress how do we treat the rot so that it doesn't spread and do further damage to the beam?'

If you can be sure of stopping the water from coming in, the fungus that is causing the rot will die once the wood dries out. However, if it would give you greater peace of mind to apply a treatment, we suggest a boron-based product such as "ProtecTimber". It dries clear, has no odour and has very low toxicity to humans, but is effective against fungal decay and insect attack. You can find out more from this website: Other brands of similar products may be available in your area.

Q. Follow-up to this recent question: I PRACTICE KARATE AND WANTED TO MAKE A "MAKIWARA" FOR USE AT HOME. IT IS A STRIKING POST FOR WHICH I NEED A TIMBER THAT IS 1) STRONG 2) FLEXIBLE AND 3) SUITABLE FOR OUTDOOR USE. IF IT HELPS, TRADITIONAL WOODS USED INCLUDE "SHIJIYA" (A MEMBER OF THE JAPANESE BEECH WOOD FAMILY), "HINOKI" (JAPANESE CYPRESS) AND "SUGI" (JAPANESE CEDAR). Your expert suggested clear radiata pine in response. Would either of hoop pine or kauri pine also be good (possibly better from a flexibility point of view)? In all cases, is it ok to have the timber cut and then treat it for outdoor use myself using some commercial coating?

Our previous answer was based on the assumption that knot-free, preservative-treated radiata pine would be easier to obtain than hoop pine or kauri pine. However, in our opinion (not being experts on karate) hoop pine and kauri pine would be similar enough to radiata pine for your purposes. Regarding treatment, if the post is under a verandah or similar covered area, treatment might not be necessary. However, if it is installed in the ground, set in concrete, or exposed to the weather, wood rot could be a problem in the longer term. In that case commercial preservative impregnation would be the most effective treatment. So we suggest you evaluate the likely exposure hazard, consider whether it is necessary for the post to have a long life, and choose a preservative treatment accordingly.

Q. I have recently painted western red cedar windows using a primer and 2 acrylic flat white top coats. The tannin appears to bleed through causing staining and discoloration, how do I remedy this situation?

Acrylic paints don't form such an effective seal as solvent-based paints, although they have other advantages. Using a solvent-based primer, followed by undercoat and enamel top coats will seal in tannin. However if you prefer an acrylic top coat try using a stain blocking primer. Dulux make one called Preplock - for more information paste this link into your browser:

Q. I am currently in the construction phase of of a pier repair project up in Northern Queensland and the contractor has asked whether F22 untreated timber can be used for the pier superstructure (headstocks, beams, corbels, deck planks etc.). Currently, the elements are specified as H6 F27 timber. Providing the elements capacities satsify their design loads, are there any other problems that would arise by using the different timber?

Preservative treatment to Hazard Level H6 is to protect timber against marine borers, ie. for piles in salt water. In our opinion it would be satisfactory to use untreated timber for the superstructure of the pier as long as it has Class 1 durability and sapwood removed. So it depends on the type of F22 timber that the contractor is offering. Durability classes for commonly used species of timber are set out in Australian Standard 5604-2005, "Timber - Natural durability ratings".

Q. The background to my previous question comes from a recent experience of exporting CCA treated hardwood. At the destination, some evidence of fungus was found on the wood surface by the local quarantine authority. (I can provide photos if you are interested.) They then insisted on some fairly high cost remedies before the wood was released. I believe that the fungus grew between sawing and treatment. (Note that we are talking about a fungus growing on the surface of the wood, not decay of the wood. Indeed, when a Forests NSW inspector saw the photo, he remarked that he would have passed that piece.) When this problem came up, I asked Harry Greaves if the treatment would kill fungus, and his advice was that it would not, hence my question on the effectiveness of CCA as a fungicide.

Whether it is misleading to describe CCA as a fungicide depends on what is meant by the term "fungicide". When wood is pressure treated with CCA the chemical is impregnated into the cell walls, making it inaccessible to the decay-causing fungi that utilise wood as a food source. In that sense perhaps CCA could be described as a fungicide. However, the term "fungicide" more correctly means something that kills fungus, so it is more accurate to say that CCA treatment protects timber against fungal decay. That is how most timber producers describe it. In other words CCA treatment prevents decay-causing fungi from becoming established in the first place, rather than killing them. As previously advised, moulds can grow on the surface of wet CCA-treated wood since moulds don't actually "feed" on wood - they need very little nutrient. If you have been in touch with Harry Greaves, then you are talking to someone with considerable expertise on the subject and you may wish to pursue the matter with him.

Q. How effective is CCA (H4) as a fungicide treatment? The chemical companies that supply it refer to CCA as a fungicide. However, when questioned, they do not claim that CCA will kill fungus that is present before treatment, nor do they guarantee that it will prevent mould or fungus growth for even a limited time after treatment. Should it really be advertised as a "fungicide"?

We weren't sure who you've been talking to, but CCA treatment carried out in accordance with the Australian Standard definitely prevents fungal decay. Generally treatment is carried out on sound new wood, rather than wood that has already started to decay. Such material would not be marketable, so the issue of killing a fungus that is present in the wood doesn't arise. However, if partly decayed wood was treated it would prevent any further fungal decay, although of course treatment would not restore the portion that was decayed. Perhaps the confusion has arisen from the fact that some moulds can grow on the surface of wet CCA-treated wood. Moulds need very little nutrient material and can grow on any damp surface, including plaster, bricks, wallpaper and so on. However, moulds do not decay wood. If you write the words "treated timber warranty" in your browser you will find plenty of treatment companies that offer a 25-year warranty against fungal decay and insect attack - some offer a "lifetime" guarantee.


Did you know?

About 16% of Australia’s 147 million hectares of native forests are in nature conservation reserves.