Most exterior woodwork needs treatment to prevent or delay rot. The main treatment options are given.
Water based Preservative
Water based preservatives have become popular due to lower cost per gallon. However this is only part of the story.
Water based preservatives have a very short life, and typically need recoating every year. This means an annual travel to the diy store, money spent, and time spent recoating the woodwork.
They are also not as effective as spirit based products, and will not effectively prevent rot in wood constantly exposed to water, such as fence post bottoms and earth retaining woodwork (eg bed edging, raised beds, compost containers).
Spirit based Preservative
Spirit based preservatives last multiple years from a single coating. This makes spirit based preservatives a much lower cost option than water based, even though the cost per gallon is higher.
This also makes spirit preservatives the more energy efficient option. Although the per gallon energy input is higher, many less gallons are used, due to less frequent application. Less transport & human travel is also needed, further reducing energy consumption. The reduced frequency of painting also means much less human time & energy input.
Oil based Preservatives
Used engine oil mixed 50/50 with paraffin or diesel has long been used as a wood preservative. It is very effective, but there are now concerns about the possible toxicity of contaminants in used engine oil.
New engine oil does not contain engine contaminants, and a new oil & paraffin mix will be discussed here.
- Fully effective
- Long lasting - many years
- Low cost - typically around £3-4/gallon
- Safe to handle
- Preserves wood in constant contact with wet earth
There are 3 drawbacks, all of which are temporary, and disappear in the days following application.
Temporary Oiliness When initially applied it creates an oily surface. Within a day this soaks into the wood, leaving a dry surface.
Flammability concerns Like any spirit based product, an oil & paraffin mix is flammable. However it can not be readily ignited like petrol. Neither diesel nor engine oil will ignite when a naked flame is applied, and nor does a paraffin & oil mix. Paraffin is more flammable than diesel, but quickly soaks in then gradually evaporates away. Wood so treated does not become an increased fire risk, the prime problem is public perception.
Whiff Paraffin creates a very mild whiff as it evaporates. This is temporary, though it lasts for many days. This makes it ill suited to indoor use.
The paraffin can be substituted with diesel, kerosene or heating oil, but not with white spirit or petrol. White spirit is not effective, and petrol is dangerously flammable & toxic.
Don't use engine oil alone, it does not soak in.
- Alkyd oil paints (household gloss paint) last a few years
- Keeps water off wood, but does not provide any anti-rot chemicals to the wood.
- Different chemistry to oil based preservatives.
Stockhom tar is smoked cooked pine resin. Tar waterproofs as well as preserves, and is thus a good choice for the bottom underground sections of wooden fence posts, and the ends of joists where inserted into masonry.
Tung oil, danish oil, boiled linseed oil etc
Creosote is the most effective and long lasting wood preservative of all. However it has downsides:
- Smelly, almost stinky
- Permanently stains clothes, carpets, paving etc
- The fumes from application kill most plants within a radius of many feet.
- Smell & plant deaths can lead to neighbour disputes
- Now banned for home use.
- May be legally applied by professionals
- Available from trade outlets, not DIY sheds
Cut ends of wood are the most vulnerable to rot due to higher porosity. Repeated coating or preferably soaking in preservative can extend life.
The more preservative you can get into the wood the longer it lasts. Soaking wood in presrvative is therefore common. This can be done with a large container, or by standing the cut ends of wood in a bucket of preservative.
It can also be done by wrapping polythene sheet under the wood, supporting all the edges above the top of the wood, and filling with preservative. This minimises container volume, and is good where a minimal quantity of wood needs treatment. Before use check your preservative does not melt the polythene.
Preservatives stain paving, so dust sheets are useful. Waterbased stains are the least problematic in this respect, as the stain soon fades and disappears.
Use of non-water based stains indoors may in some cases produce concentration of fumes high enough to be an immediate risk to health. Check the label.
Some woods dont need preservatives for outdoor use. These include: