Preservative treatments & finishes

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Question: I am designing a studio that is intended to have KDHW timber window and door frames. We would like to achieve a natural timber finish rather than paint it. What is the best varnish product for this outcome considering that it will be exposed to exterior weather conditions.

Answer: The most important factor is to choose a varnish that is formulated for external conditions and contains ultra violet (UV) absorbers. Interior varnishes become brittle and will crack and peel after exposure to the sun. Even exterior varnishes need monitoring and maintaining at the first sign of failure. Any shade you can provide via eaves overhangs, window hoods, foliage, etc. will help to prolong the life of the finish. A number of companies market exterior varnishes and we have not tested them so we are not in a position to recommend the "best" product. Nevertheless, Sikkens products seem to have a good reputation.


Question: Should I specify weathering before application of decking oil to a new spotted gum deck? Would you recommend oil or water based for the best long term solution?

Answer: 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. 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. If it's a hardwood that is likely to leach tannin, 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. Apart from that, we don't think there's any need to delay coating it. With regard to water based v. oil based finishes, we think they are equally suitable as long as they provide equivalent water repellency. This can be tested by observing whether water "beads up" when it strikes the coated surface. Of course, it's easier to clean up brushes and rollers when using a water based finish.


Question: We have built an outside seating area with kapur and stained it with decking stain Cabots...then sealed the top and sides with decking oil. The stain is now running all over the supporting pavers and looks terrible. How to fix?

Answer: We weren't sure if the stain you refer to is the decking stain washing off or a tannin stain coming out of the timber. Decking stain doesn't usually wash off once it has dried, so presumably it's tannin staining. Most hardwoods, including kapur, contain tannin which washes out of the wood on exposure to rain, leaving a brown stain on adjacent surfaces. Cabot's recommend a preliminary scrub with their deck cleaning product to remove some of the tannin from new hardwood, so it might be worth trying that. You could phone the Cabot's help line to see whether they would recommend it at this stage. The only other options are to seal the wood with paint or simply wait for the process to run its course. Tannin stains can be cleaned off masonry with oxalic acid, or a proprietary cleaner that contains oxalic acid.


Question: House which is constructed out of western red cedar, could you please advise what to use to re-seal the outside as it has gone black/grey colour? The decking needs re-sealing as well? Also what to use on the window frames?

Answer: It can be a lot of work keeping the natural colour of wood outdoors. The traditional weatherboard house was always painted, which allowed quite long maintenance intervals. Clear oils and varnishes have a shorter life than paint as they are more susceptible to ultraviolet attack. They are still a practical choice for relatively small areas, but perhaps a bit demanding when your whole house is clad in timber. When a natural look is desired, we feel the best compromise is to use a semi-transparent stain. These finishes achieve a longer life than clear oil and don't fail like a varnish by peeling. We suggest you give your cladding a good scrub with one of the colour restoring products to remove the grey weathering, and then apply a semi-transparent stain in a light cedar tint. Depending on how much decking you have, you might be prepared to keep up a clear oil finish. Otherwise the choices are a pigmented decking stain, or to let it weather off to a grey tone. Regarding your windows, people often use paint to give a colour accent in contrast with the stain finish on the cladding. Paint is particularly useful if the glass is installed with putty, since putty needs the protection of a paint coating to prevent it from drying out and cracking. On the other hand, if your glass is installed with wooden glazing beads a stain finish will be satisfactory. Note that this is general advice and we don't know how large your house is, whether it is one or two storeys, or what budget you are prepared to commit to maintenance. On a two storey house, it's usually desirable to use long-lasting finishes because of the greater difficulty of access, but this depends on individual preference.


Question: Hi, I would like a pale weathered grey appearance for a timber deck. We have chosen spotted gum. Should the underside be coated with anything? It is being laid above a concrete base.

Answer: Spotted gum is a good choice as it is a Class 1 Durability timber when used outside above ground. We don't consider there is any need to coat the underside as long as there is plenty of air flow under the deck. You mention the decking is to be laid above a concrete base. We have seen problems where timber decking is laid without much clearance underneath, so make sure there is a fall on the concrete to allow rainwater to drain away. If water is able to pool on top of the concrete it may be absorbed into the underside of the decking, causing it to swell and/or cup. It's also not recommended to enclose the sides of the deck - although it gives a neat finish, enclosing the deck cuts off the flow of air and allows humidity to build up under the deck.


Question: How long do you leave merbau decking to bleed before oiling?

Answer: Merbau decking won't bleed on its own. When people talk about it bleeding they are referring to the tannin that washes out on exposure to rain. If it doesn't rain, or the decking is sheltered, it doesn't bleed. Assuming your decking is exposed to the weather, the time it takes for the tannin to wash out will depend on how much rain you have in coming months. To hurry up the process, manufacturers of decking oil recommend a preliminary scrub with deck cleaner. This tends to bring the tannin to the surface. Using deck cleaner to remove tannin is not something we have tested, so we make no claim that it will completely stop bleeding, but you might wish to discuss it with one of the makers of decking oil. The alternative is to leave the decking for some months so the tannin can wash out naturally, but in the process it will start to lose its colour and turn grey. So if you want to keep the colour, scrubbing with deck cleaner, allowing it to dry, and then applying oil would seem the best option.


Question: I am building a new deck with spotted gum 136 x 19mm. Intergrain natural stain has been advised over Intergrain natural decking oil. I prefer the natural finish but will I be risking the stability of the timber?

If your deck is fully exposed to the weather it will be subject to wetting and drying cycles as the weather changes. To the extent that a finish excludes or repels moisture it will have a stabilising effect on the timber, reducing the tendency to swell and shrink by limiting moisture absorption. The most effective coating is a paint film, since it forms a skin over the wood. However, it is not generally acceptable to paint a deck - apart from aesthetics, the paint wears off under foot traffic. Oil coatings repel moisture and help to retain the natural colour of the wood, which without a coating will turn grey. The water repellent qualities of oil finishes can be observed after rain when the water forms globules on the surface of the wood, rather than soaking in. The density of spotted gum also means it does not absorb significant amounts of moisture from short-term wetting, but only when exposed to moisture for prolonged periods. So you are not risking the stability of your decking by using an oil finish, particularly if you are prepared to re-apply it regularly.


Question: We are building a hall for a school with poles encased in concrete footings 2m deep in the ground, the poles are red ironbark narrow leaf. Can you pl. advise of alternate method of protection other than chemical for termite protection?

Answer: Red narrow-leaved ironbark is a highly durable timber, and is rated Class 1 Durability for in-ground contact according to Australian Standard 5604-2005, Timber - Natural durability ratings. It therefore has a high natural resistance to wood rot and insect attack without any treatment. Encasing timber in concrete may or may not have a beneficial effect, depending on whether the concrete is porous and retains moisture, or is dense and excludes moisture. A non-chemical method of termite protection is a stainless steel "sock" fitted to the in-ground portion of the poles. Alternatively, inserting boron sticks into the base of the poles will provide added protection against decay and insect attack. Although not strictly "non-chemical", boron has very low toxicity and by this method is retained inside the pole. The technique is used by utilities to maintain their service poles.


Question: What is the best way to preserve hardwood? What are the merits of oils and clearcoats? Do you recommend neither, one or both?

Answer: Both finishes have their advantages and disadvantages. Oils are easy to apply and can be re-coated with little surface preparation, but generally don't last as long as varnishes. On the other hand, varnishes are more laborious to apply and may fail by cracking and peeling if not maintained. Of course, the level of exposure plays a big part. Timber under a verandah or pergola will need much less maintenance than timber that is fully exposed to the weather. If you decide on a film-forming coating (varnish), pick one that contains UV absorbers and has a good track record in the Australian climate. Whether you choose an oil or a varnish, it's worth considering a product that contains some pigment, rather than one that is perfectly clear, since this also helps to protect against UV. If you would like to discuss this further with an advisor, phone the National Timber Information Line on 1902 28 2000.


Question: Merbau decking finish for seaside surf beach extremely strong winds have used finishes that deteriorate after 12 months.
Answer: The usual range of oils and decking stains don't penetrate deeply into hardwoods such as merbau. The qualities that make hardwoods good for decking (durability and resistance to wear) also mean there is little absorption of coatings. Consequently, in exposed locations finishes tend to wear off fairly quickly and you might have to resign yourself to an annual maintenance program. Decking paint will last longer than the more natural-looking oils and stains, but decking paint completely covers the timber grain. Also when maintenance is needed it's a bigger job than just applying more oil or stain, particularly if the deck paint reaches the point where it starts to peel. Maybe the best option would be to leave the deck to weather to a driftwood grey and forget about maintenance. Merbau has good weathering properties and if you are 5 mins from a surf beach you've probably got better things to do!


Answer: There are several manufacturers of LVL in the Australian market, and you should be able to obtain details of their warranties directly from the relevant companies. As an example, Hyspan LVL with an H2 glueline treatment carries a warranty of 25 years against termite attack, subject to certain conditions. This does not necessarily mean the treatment ceases to be effective after 25 years, but that is the maximum period for which the company is prepared to accept liability.

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