Painting will help to preserve the surface and therefore the interior cells of the wood you use, particularly if it is done properly, but paint alone will not achieve your goal. There are two main causes of deterioration of wood: decay and insect damage. Your problem is to outwit both of them, perhaps in one operation, as cheaply and easily as possible. Many woods, such as redwood, cypress, locust, and cedar, have natural qualities of resistance to bacteria and even to insects, but most woods of the less expensive grades (and some of the more expensive as well) are susceptible to these twin causes of disintegration. The naturally resistant woods do not break down so soon and permit rot to enter; they resist penetration by termites and other insects, or are immune to them even when in contact with the soil or placed in areas of constant dampness. Other woods left untreated, however, will soon succumb in such situations.

Formerly, creosote was the only effective means of preserving wood. It was used extensively on fence posts, trellis posts, and on other wood which was to be in constant contact with the soil. It is still widely used, and you can buy posts already treated with creosote under pressure, the treatment forcing the preservative deeply into the pores of the wood. However, it is not easy for the amateur to deal with creosote. Brushed on, it is not very effective; and soaking achieves penetration best only by using hot creosote and immersing the wood in pressurized vats, something which only large industrial plants are equipped to do. There are some disadvantages, too, in its use on wood structures in the garden by the amateur. The fumes and gases of creosote permeate the soil around the post and, unless the post is set in concrete to the level of the treatment, may injure or kill the roots of shrubbery or vines planted close to the post.