You can protect natural wood from inevitable deterioration in many ways. Paint stores offer wood finishes for indoor and outdoor use, but by taking the time to mix your own homemade wood preservative you can control the ingredients. You can also be sure you have a product that is tailored to the needs of your project and is to non-toxic and environmentally friendly. Very likely you’ll save money in the process. Some research will be required; there are dangers to be avoided in the ingredients you will choose for your mixture.
Consider the Environment
Any wood in contact with the ground will deteriorate rapidly, courtesy of water. Fenceposts are particularly vulnerable since they are sunk into the ground, but anything made of wood, such as sheds, fences and planter boxes, will need to be protected from moisture. Mixtures for outdoors will probably need to include some toxic ingredients to protect from moisture, rot and bacteria. Indoors, wood is not threatened so much by these dangers, but environmental conditions in even the cleanest rooms will eventually cause the wood to discolour and start to rot.
Linseed oil, which is extracted from flax seeds, is a traditional preservative. It is natural and nontoxic. The disadvantage is that it dries slowly and tends to be sticky. “Boiled” linseed oil dries more quickly but has some additives to speed the drying. Check out the Natural Handyman website (see References) for a detailed discussion of linseed oil and its uses. Since linseed oil attracts mildew, you may have to consider a mildecide, which you can buy in packets at a hardware store.
An alternative that’s also natural, but expensive, is tung oil that is made from the nuts of the tung tree in China.
Resins can be used in the mix. Pine tar is a natural resin and there are alkyd resins that imitate it. They make the finish harder and more weather resistant.
Other ingredients may include turpentine and/or paint thinner. Turpentine has a very strong odour, so be sure you use it in the open air or with good ventilation. Paint thinner is considered a good and efficient alternative. If you’re going as natural and nontoxic as possible, you might want to consider a “green” solvent made from citrus oil.
You can buy borates at your neighbourhood supermarket under the name 20-Mule-Team Borax–the same one you might use to boost your washing powder. It's safe and nontoxic.
The same antifreeze you use to keep the windshield clear can be used as wood preservative. But make sure you are getting propyline glycol, not ethylene glycol, which is extremely toxic to almost anything. Don’t try this mixture without consulting the extensive information at the Bearfort Lodge website (see References). In its "Tags" section, under "wood preservatives," is a serious discussion of the advantages and dangers that will help you decide if you want to go this route.
Creosote has been used as a wood preservative for many years, but it ignites easily and is now banned. A heated mixture of pine tar, creosote and linseed oil used to be painted on to bare wood to preserve it, but in the United States and Great Britain as well as many other countries it is not longer permitted.
Obviously you wouldn’t want to use water soluble preservatives outdoors since they leach out of the wood in the long run with exposure to water and moisture.