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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. We have just had our 100 year old Kauri Pine floors sanded and finished in a 2 pac water based polyurethane. The sanding is fine and the finish is just as we hoped, however the timber itself appears quite ‘blotchy’ in places (aside from the natural timber variations) and we were told by the floor contractor that this is normal for Kauri Pine. The blotches are very pale (although not milky) and were evident after the initial sealer was applied. They are more prominent in high traffic areas but not all and are of varying sizes and shapes, ie not consistent with the look of previous stains (no evidence of previous stains after sanding). Any ideas on what is causing these blotches?

We weren't sure whether your floor was stained (ie. a colour was applied before the polyurethane) or whether a clear polyurethane went straight onto bare wood. Wood stains aren't always evenly absorbed by softwoods, and sometimes the effect is blotchy, since the more porous areas absorb more stain. Something of the same thing can occur with polyurethane. If there are patches of highly absorbent wood, it's possible that the finish is being sucked up instead of building up to an even film on the surface. It's also possible that traces of a previous coating remain in the wood and are interfering with the new coating. Perhaps another coat of finish would help (you don't mention how many were applied) or perhaps it will be necessary to lightly sand and start again. It's also possible that the polyurethane manufacturer could advise you further. Most of the major companies have a customer help line.

Q. I bought a new treated pine cubby. I want to stain it before putting it up. It is in panel/kit form. It is treated pine timber, with some of the blue treatment coming through. I have read many forums and spoke to the suppliers of stains, and get different answers. Do I need to let the treated pine age/weather before staining? I would like to stain before erecting as where its going, it's hard to get to and won't weather evenly. I was thinking of using dexpress or intergrain natural stain.

We assume the timber is actually green rather than blue. "Blue Pine", which is marketed for house framing, is not suitable for exposure to the weather. There's no need to let the timber age or weather. It's sometimes suggested in the case of dense hardwoods so that they become more absorbent and the finish is better able to penetrate the wood. Kiln-dried pine is already an absorbent material so pre-staining the cubby before erecting it won't be a problem.

Q. I have a very weathered Sydney blue gum outdoor setting. It has been treated with decking oil every few years, never often enough. The guy that made it recommended using a mix of polyurethane and decking oil but I have now forgotten the recommended proportions. Have you heard of this and what proportions should I use. I think it was more oil than poly.

Mixtures of oil and polyurethane have been marketed from time to time, eg. Watco which might be available in your area. For DIY purposes, one reference suggests a mixture of one part oil, one part varnish, and two parts turpentine. This isn't something we've tried or tested, so we don't make any claims about its effectiveness. However, you might like to experiment with various mixtures, using this as a guide. The purpose of including polyurethane is to create a finish that lasts longer than straight oil, but we emphasise that we haven't done any comparisons. A good quality decking oil might be just as good.

Q. I am hoping that you can help me decide what to coat/treat the teak wood (rather, it LOOKS like teak to my woodworking friend!) that I wish to erect on my roof peak. Advice so far from amateur friends includes; 1) Coat it with "Teak Oil" that can be purchased in small cans. 2) Soak it several days in linseed oil. I am afraid that neither of these ideas will result in the MOST permanent treatment, I wish for this as after installation high up on the roof they will be very difficult to re-coat in subsequent years. Please see the pictures of some similar outdoor cabanas in Thailand. I NEVER see any 'peeling' on these as I travel around there, and wonder what YOU think they do to get such a rich, shiny surface that hardly seems to suffer at all in the weather. To me, it 'looks' like some kind of varnish, which most here say will indeed peel and flake off after time. Too bad I did not speak enough Thai to get advice when I was there. Any ideas you could give me for a good coating will be much appreciated.

You could send photos to, but even if we saw the photos it wouldn't tell us what kind of coating they use in Thailand. It's hard to come up with a natural coating that will last a long time in the weather, since varnishes eventually crack and peel, although some last longer than others. Teak oil is probably intended for outdoor furniture where you can re-coat it whenever it needs doing. Maybe in Thailand they just climb up and oil their timber frequently. We wouldn't recommend soaking it in linseed oil. Linseed is inclined to turn black in the weather due to mould growth. Probably the longest-lasting treatment that won't peel is an epoxy soak. You will find information about this on the net if you write WEST system in your browser. WEST stands for wood-epoxy saturation technique, and it's used in boatbuilding.

Q. I am a structural Engineer designing some external covered walkway roofs and we are looking at framing these out of timber. I was considering a seasoned MGP10. The covered walkways are now mostly open and the soffit of the roofs will no longer be lined, leaving the timber exposed except for the roof sheeting. The timber would need protection for a H3 Hazard classification so an MGP10 would need to be treated. The architect is concerned now (now that the roof framing will be visible) that MGP10 can sometimes come with fine grooves along the length of the grain, which I assume is from the way the timber is cut. I have seen this in wall studs though not in larger members (such as 140x45 to 290x45 which we need for this roof). Can you please advise whether this is common in MGP10 to have these 'grooves' along the side of the timber? Or would another grade of timber (perhaps a naturally durable hardwood or a cypress pine) be better suited for this application?

Some timber mills produce timber with a "micro reeded" or ribbed surface, particularly on treated pine products. This is explained in more detail on this link: However, we wondered if it will be strictly necessary to treat the timber if it is under a roof. The test is whether it will get wet when it rains, rather than simply whether it is indoors or outdoors. If it will be exposed to rain then of course pine must be treated. Perhaps the most suitable pine product would be LOSP-treated pine. This has an H3 rating and the better products come pre-primed ready to paint, eg. Design Pine. It is also produced from knot-free material giving a higher quality appearance, suited to architectural applications.

Q. Can you please provide me information in relation to the design life of timber treated to different hazard levels. I downloaded 05-Timber service life design from your website; Page 32 of the document gives 40 years design life for zone-C and H3 treatment. When I speak with supplier they said the design life for H3 above ground would be about 20-25 years. Can you please advise?

Looks as if you are investigating treated pine for a fence. The figures given in Design Guide 05 "Timber service life design" are reliable since they are derived from an extensive research program. Having said that, they indicate a "typical" service life and individual pieces of timber, or the fence as a whole, may achieve a slightly longer or shorter life depending on the specific situation. For example if the fence happened to be in the path of an automatic watering system, that might have an effect on the life of the timber. Note that the 40-year prediction also assumes the timber will be treated in accordance with the Australian Standard. It's possible that the discrepancy between your supplier's advice and the guide arises from the fact that some timber producers give a 25-year warranty on treated timber. Other suppliers and producers give warranties up to 40 years. You will find details of supplier warranties if you write "treated pine warranty" in your browser.

Q. Would 150 x 150 cypress pine or ACQ treated pine posts be more durable where used as in-ground fence posts in concrete footings?

Cypress pine heartwood is durable in ground contact and has a high resistance to termite attack. However, we would regard ACQ treated pine to be more reliable, as long as it is treated in accordance with the Australian Standard. The size you intend to use is quite large, and cypress pine often comes from relatively small trees. This means 150 x 150 posts could contain sapwood (the lighter-coloured outer wood), which is non-durable. When using treated pine try to make sure the uncut end goes into the ground, if the posts have to be cut to length, as this will be the most heavily treated zone.

Q. We would like to paint the particle board flooring at our beachhouse white. Is this ok and what paint and finish would you suggest?

When contemplating painted flooring, be aware that the paint will tend to wear off in high traffic areas, particularly if people walk in with sandy feet. Rugs or mats in strategic spots will help. If you decide to go ahead there are a couple of options. You could apply an opaque wood stain and seal it with flooring grade polyurethane, making sure the products were compatible. Or you could paint it with flooring paint, such as Porter's Perfect Floor Paint. For more information, visit Note that this is just a suggestion - we have not used or tested this product, and have no information about its suitability for particleboard.

Q. What is the expected life span of H2F timber treated with permethrin? Please send scientific data to substantiate the durability of this system. Thanking you in advance.

The question really is "what is the life-span of permethrin" - the timber will last indefinitely in a protected indoor situation, unless it is attacked by termites. We do not have data on the persistence of permethrin when applied to wood that is subsequently located in a protected situation, ie. a house frame. However, before H2F timber was introduced to the market it was tested in the field, with envelope treatments of permethrin and deltamethrin, and found to be effective against termite attack. The test program is described in a paper by Brenton Peters and James Creffield, published in the US Forest Products Journal, ref. Forest Prod. J. 54(12):9-14. Producers of H2F timber (marketed as "Blue Pine") offer a 25-year warranty against termite attack and as far as we are aware there have been no claims made to date. Nevertheless, we encourage builders to install barrier systems as well as using Blue Pine, so that the H2F treatment can be regarded as a second line of defence, rather than the sole defence against termite attack.

Q. We would like advice in relation to a product that would be able to be applied to external timber to achieve an H3 standard. The product is to be applied to following timber: - Bearers: 175x75 F8u HW - Joists: 90x45 F17 SEASONED HW.

If the timber in question is of natural durability class 1 and free of sapwood, treatment is not required in order to achieve H3 level. However, if the timber is of natural durability class 2, 3 or 4 the preservative has to penetrate all sapwood plus into the heartwood to a specified depth, as set out in AS 1604. This is unlikely to be achievable with a surface coating - in fact it's difficult enough to penetrate hardwood with a pressure treatment. So we doubt that there is a product that will meet your requirements.


Did you know?

About 6% of Australia’s 147 million hectares of native forests are public forests potentially available for timber harvesting. Timber is harvested from about 1% of those public native forests each year.