Ask an Expert

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. We have Exterior Intergrain Clear on our window sills and doors. We don't want to continue using it; are there any varnish products that will go over it.

If the Intergrain coating is in good condition, and hasn't started to weather off or peel, it should be possible to paint over it with a product of similar formulation. However, to be absolutely sure it would be best to continue using the same product. If the Intergrain product has not been maintained and is starting to fail you will need to remove it before starting with a new finish. To do this, Intergrain recommend using Intergrain Liquid 8 Paint Remover and power washing, or alternatively sanding.

Q. I would like to know the grade of treated pine to use if I sink treated pine posts into a concrete foundation. This is for a fence project.

Your treated pine posts must be at least Hazard Class 4 for in-ground use. We also prefer to see post holes backfilled with rammed earth or fine crushed rock, since this permits better drainage. If concrete is required for the stability of the fence, then "no-fines" concrete is recommended, ie. coarse aggregate only. A definition of no-fines concrete can be found on the net.

Q. I am working as the engineer designer on a repair project on the Queensland Coast on a Pedestrian Pier. Currently we have specified timber that to be used is Class H6 and Stress Grade F27, however the contractor is having lead time problems with sourcing such timber and has suggested that Class H5 F27 timber would be fine. I have read through various codes and am suspicious whether using Class H5 timber would appropriate as AS 1604.1 Appendix D suggests that for Jetty elements such as this project, Class H6 timber should be chosen. If the contractor were to proceed with Class H5 timber, what durability implications could occur?

The essential difference between H5 and H6 treatments is that the former is intended for timber in fresh water (or for critical structures in ground contact), whereas the latter is intended for timber in salt water. It's not that salt water in itself is particularly hazardous - the higher level of treatment is to provide protection against marine borer attack. If the jetty elements are all above water we would consider H5 to be satisfactory, but if the timber is in the water (eg. jetty piles) H6 should be specified - unless, of course, the structure is temporary or designed for a relatively short life. Marine borer attack is more aggressive in the northern waters of Australia, ie. north of Bateman's Bay.

Q. Can you please advise who/what company can impregnate a fire retardant chemical into timber to give a result of either Material Group 1 or 2?

Worthington Industries offer a fire retardant impregnation treatment and you can visit their website at for more information. We understand their treated products are limited to indoor use, and no doubt Worthington can provide a test report to show which Material Group their treatment falls into. It's also possible to obtain fire retardant treated MDF overlaid with a wood veneer which will meet Material Group 2 requirements. For information on treated MDF refer to the Timber Veneer Association of Australia (TVAA) website at

Q. I have been offered hardwood poles that are DOUBLE DIPPED H5 for use in a lightly salted lagoon. Would that be sufficient? If not is there anywhere in Brisbane area that does H6?

The term "double dipped" may mean that the H5 level has been achieved by putting the timber through the treatment process twice, which may or may not meet Australian Standard requirements. It's slightly misleading in that the treatment is not a dip treatment, it's a pressure treatment (or should be). Also timber treated to H5 for house stumps and building poles is normally treated with CCA. Experience has shown that in the northern waters of Australia CCA treatment alone is not effective against marine borers. Effective treatment in northern waters requires "double treatment" which means CCA plus creosote. We suggest you inquire from Koppers about the availability of their H6 treated piles. They also have an informative data sheet titled Timber Piling Life Expectancy which you can find if you write "double treatment H6" in your browser.

Q. We have a deck around a pool which is built with sleepers. The sleepers have gone grey and need to be revived. Could you pls suggest what would be the best to coat the sleepers with to revive them and give them some colour. We are currently looking at intergrain, sikkens and cabots products. Obviously being around a pool needs to wear well.

There are a couple of ways to approach this. You could choose a heavily pigmented exterior wood stain and apply it directly to the grey timber. This will give the wood a solid colour with little wood grain showing through, but avoids the chore of trying to restore the natural colour. If you want a more natural wood look, with a decking oil finish or a lightly pigmented stain, you will need to get rid of the greyed surface first. This can be done by scrubbing the sleepers with a proprietary wood cleaning product, such as Intergrain Reviva, Cabot's Deck Clean, etc. Or if the sleepers have a slightly roughened surface it might be easier to give them a blast with a high pressure hose. If you write "pressure cleaning wood" in your browser you will find lots of information on the subject, including reports from people who got it slightly wrong.

Q. I have wood fence palings that are treated to H3.2, some of them will be in ground contact for about 600mm. What could I treat them with to prevent them from rotting?

We take it that Hazard Class H3.2 refers to the higher level of LOSP, recommended for structural products in exposed situations above ground. AS 1604 doesn't actually use the term "H3.2", but it does provide for two levels of H3 treatment. However, H3 treatments are not intended for in-ground situations and it's doubtful they will provide adequate long-term protection when placed in ground contact. There are plenty of fungicides on the market that you could apply by brushing on or, better still, by soaking the ends of the palings - however, these preservatives are not likely to remain fixed in the wood and it would be far preferable to keep the palings out of the ground. Alternatively, palings treated with CCA might achieve a longer life in this situation.

Q. I've got some solid (pine?) interior doors from my 1923 Queenslander style home which were chemically stripped approx 2.5yrs ago. They've been stored in a ventilated, but possibly still moist area under the house, on concrete. Over the storage period the've always appeared and felt quite moist with visible moisture appearing at the endgrain where it butts up to timber. I've painted a test patch using an oil undercoat to see if the moisture would affect the paint. after about 5-6 days, little beads of brown residue (presumably tannins) formed on top of the undercoat. Could you please advise on a reliable method or treatment to prevent any problems after painting (intend to use an oil based gloss topcoat)? Should the doors be dried prior to painting and what's the best method of achieving that?

If your doors were stripped in a caustic solution they would have soaked up a good deal of moisture, since caustic dips are water-based. After two years they should have dried again, assuming they were stacked clear of the concrete and separated from one another. Perhaps the watermarks adjacent to the end-grain have been there for a while and appeared soon after dipping. It's hard to say what has caused the beads of brown residue - they could be shellac or wax left behind from the original coating. If so, it's possible that a solvent-based primer is re-dissolving some of the waxy residue and bringing it to the surface. The brown beads are unlikely to be tannin, since softwoods are low in tannins. You will find some discussion about this kind of problem if you write "caustic dip brown residue" in your browser. Whatever the cause, we suggest you try a stain-blocking primer, rather than a conventional primer. Major paint companies have customer help lines and they might also be able to advise you.

Q. Is there a decking treatment that gives colour and doesn't have to be re-applied every year?

The finishes that last the longest are those with the most pigment, so decking paint will generally give the best results, depending on foot traffic. However, paints obliterate the grain so it's a trade-off between keeping some of the natural wood look and reducing maintenance intervals. The Flood Company, manufacturers of Spa-n-Deck, offer a 2-year guarantee on their product so that might be worth trying. However, we emphasise that haven't tested any coatings and don't endorse any particular products.

Q. I have just stained my brand new pine deck with a water based stain and it is too orange? It's a large deck . What can I do? Will a darker stain cover it?

It's possible to stain wood a darker colour, it's staining wood a lighter colour that's more difficult. However, it might take a bit of trial and error to get the right effect. If you have any offcuts it would be best to try out some stains on those, rather than the deck itself. Sometimes paint stores are prepared to stain up samples for customers, so you might want to discuss it further where you bought the original stain.


Did you know?

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