Wood Preservatives

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Work in progress


Most exterior woodwork needs treatment to prevent or delay rot. The main treatment options are given.


Preservatives

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

Some 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.


Paint

  • 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.


Tar

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.

Expert article on Tar


Liquid asphalt

A sprayable diluted asphalt.


Oils

Tung oil, danish oil, boiled linseed oil etc


Creosote

Creosote is the most effective and long lasting wood preservative of all. However it has downsides:

  • Smelly, almost stinky
  • Filthy
  • Permanently stains clothes, carpets, paving etc
  • Toxic
  • 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, but not DIY sheds


Creocote

Not equivalent to creosote.


Application Tips

Thirsty Ends

Cut ends of wood are the most vulnerable to rot due to higher porosity. Repeated coating or preferably soaking in preservative can extend life.

Soaking

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.

Dust sheets

Preservatives stain paving, so dust sheets are useful. Newspaper absorbs spilt preservatives and helps reduce spread due to treading. Waterbased stains are the least problematic in this respect, as the stain soon fades and disappears.

Indoor use

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.


Durable Woods

Some woods don't need preservatives for outdoor use. These include:

  • Oak


See Also