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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 was just wondering, if I was to oil my new deck flooring with Cabot's Natural Decking Oil does the Timber have to sit for the 4 to 6 weeks duration (if the Timber has been Kiln Dried) before any Product is applied?

There are two schools of thought about allowing decking to weather before coating it with oil. Some say it should be weathered for a time so the oil 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. However research bodies have been unable to duplicate mill glaze in the laboratory. You will find more about this on the net. Others say timber should be coated as soon as possible, before the natural colour starts to weather off to grey. We are inclined to the latter view, ie. coat it straight away, particularly if your decking is treated pine. Perhaps you should ask the paint store why they gave this advice in case there is something we are unaware of.

Q. We are working on a project which will incorporate the use of hardwood within the marine environment and we have a query regarding the treatment of this timber. The work is for a high profile job within Perth incorporating timber decking on 2 jetties and a timber decked terrace which would be inundated during high water levels. The hazard class would therefore be H6 in accordance with AS1604.1-2012. The structures have a design life of 50 years, so a durable timber is required. The contractor is looking to use recycled Spotted Gum and Iron Bark. We had specified treatment of the timber in accordance with AS1604.1 for Hazard Class H6. However, the contractor has stated that market feedback is that it is not industry standard to treat hardwoods, and a treated hardwood will not achieve an H6 rating because the treatment will not be able to penetrate more than a few mm into the wood. Is this true and if so what is recommended for ensuring adequate durability of the timber is achieved?

It is correct that the heartwood of dense Australian hardwoods is difficult to penetrate by conventional pressure preservative treatment. In the past special high pressure treatments were trialled for hardwood railway sleepers, sometimes assisted by incising, but we are not aware of these techniques being used today. The only point in treating dense hardwoods is if they are used in the round with the outer sapwood band intact. Many species have treatable sapwood which provides a protective envelope. In the case of recycled sawn timber, the structure would be relying on the natural durability of the timber for protection. Salt water is not particularly hazardous in itself - the deterioration of durable hardwoods in a marine environment is largely due to marine borer attack which occurs in timber permanently immersed, such as jetty piles. If the decking is only inundated occasionally we consider marine borers are most unlikely to become established since they can only survive out of water for a short time. That is why marine borer attack in jetty timbers does not progress above the water line. While it appears the timber will be mostly above the waterline, it will be a very aggressive situation for metal fasteners which will need to be chosen with care.

Q. We own a western red cedar cottage in the Blue Mountains of NSW and are having GREAT difficulty finding a tradesperson willing to take on the big job of a 5 year maintenance regime. We used inter-grain first time but this has worn very badly on western and northern sides of house. It will be very sad if we have to paint..because there are not the trades persons with skills to do the job. Because it is 2 storey... we do not have the scaffolding or tools for the job. Do you have a list of tradespersons or companies that DO timber maintenance ?

We don't actually have a list of painters but we understand that Country Painting are happy to work in the Blue Mountains area. When we spoke to them today they indicated they would be able to work on a two-storey western red cedar house and they are familiar with the Intergrain product. For more information phone Douglas on 0400 968 254, or visit their website at this address:

Q. Our home Has WA Blackbutt floors and we have now had them re-sanded and had Bona Traffic sealer. Can you confirm the time required before we move back in. We have been told 2 days to move furniture in but not to put down rugs etc ? Possibly 5 days to fully cure. We would prefer to wait until it has totally dried. What would you recommend? Also, if a lower level that has been finished but does need to be re-done due to dirt being blown onto the wet seal, would this again require the full drying time again, before furniture is placed back ie 5 days or more?

Bona advise that their finishes dry in 2-3 hours, but take 7 days to "cure", ie. to reach full strength and wear resistance. There should be no problem walking carefully on the floor after 2-3 hours, but we wouldn't advise moving your furniture or rugs in until the 7-day period has passed. If the lower floor is being re-done, that's the same as starting again and the same curing time applies. For more details, refer to Bona literature available via this link:

Q. The bleaching effect is what we are after. For how long do you leave the caustic on the timber? The next step important would be "washing" the treated floor timber with water is to remove the excess caustic and extractives. Is there a trick as to how you do that?

We weren't sure how much you want to lighten the wood, but the professionals generally use a two-stage process. The first stage is an application of caustic soda (sodium hydroxide) and the second stage is an application of hydrogen peroxide. There's a good explanation of this on the net at this address: It is important to wash the floor after bleaching to remove any residue. Some authorities suggest a 2 per cent solution of white vinegar (ie. very mildly acidic) to neutralize the caustic.

Q. Does washing Blackbutt flooring with say a 0.1% caustic soda solution remove enough "extractives" to improve the adhesion of a urethane coating? The wood colour also appears more uniform and less pink.

A more usual strategy for removing greasiness and extractives is to wash the wood with a 10 per cent solution of sodium hydroxide. Caustic soda is likely to cause slight bleaching. Having said that, extractives are more commonly a problem with gluing rather than coating. Seasoned timber used indoors doesn't usually present a problem for urethane coatings.

Q. I am seeking a small quantity of biocide to eliminate possible dry rot in the deckhead of my vintage wooden boat which has an epoxy skin over the original timber 'ceiling' boards. My research indicates that Borates, delivered by capillary action, could address the problem. Are you able to point me towards a commercially available product or company?

We suggest the borron-based product ProtecTimber. Boron is an effective anti-fungal agent, but fairly harmless to humans. Oddly enough the timber doesn't have to be dry before you apply the product. In fact if the timber is dry it's recommended to wet it to assist the uptake of the preservative since boron is mobile in moisture. Further information is available on the manufacturer's website at

Q. I’m trying to stain an external plywood wall so that it ends up strong dark blue with the timber showing clearly. Ive added colour to polyurethane before to get strong pinks but never blues and greens, is there a product which I can buy to achieve this?

Most exterior wood stains are produced in natural timber tones in the expectation that users will want to retain the "woody" look. However, there are a limited number of stains in primary colours - maybe you can find something in the Sikkens range that would suit. Failing that, a paint shop should be able to tint a decking oil product to the shade you need. If you do it yourself make sure the tint is compatible with the decking product - some decking "oils" are actually water-based. We feel an exterior wood stain or oil is preferable to polyurethane since the latter is inclined to fail by cracking and peeling in the longer term.

Q. Can you tell me the difference between Loba and Bona Traffic HD - the Water Based products, AND the difference between 1 & 2 Pak products for each of these companies.

We are not familiar with Loba but apparently it's similar to Bona, ie. water-based polyurethane. Either system should be OK, but like all coatings a successful outcome depends to some extent on the skill of the operator and their familarity with using the product. Maybe whoever you select to do the job could show you some previous work - that's always a good guide.

Q. We are in WA and have WA Blackbutt Flooring. We originally had it coated with Polyurethane 2 Pak finish, but are about to have it re-done (14 years later and lasted very well although there are milky footprints of Tradies Boot marks coming through ??). 1st Question: Why would the footmarks start to show through in the last 18 months - 2 years. What causes this 2nd Question: the floor people are suggesting that we go for another finish to Polyurethane as it takes longer to cure and also is carcinogenic and the smell will stay in the house for approx. 3 months . They are suggesting a product Bona Traffic HD. Can you tell me about this product pros and cons and is this correct about the Polyurethane. We will be moving out completely whilst this is all done and I need an answer URGENTLY. THANK YOU

In answer to your questions, (1) The footmark problem sounds like "ghosting", a weird occurrence that is described in a special data sheet prepared by the Australian Timber Flooring Association (ATFA) - for more detail, paste this link into your browser: (2) It's only solvent-based polyurethanes that contain isocyanates, and it's the isocyanates that have caused cancers in laboratory animals. Bona Traffic, which has been recommended to you, is a water-based polyurethane and therefore isocyanate-free. More detail about the product is available on the net. Having said that, solvent-based polyurethanes are freely available and we are not aware that they are harmful to the user if recommended precautions are taken.


Did you know?

A government report showed there is no evidence proving that harvesting timber from native forests has reduced overall forest biodiversity or led to the extinction of any species of plant or animal.