Why
Seal Wood?
Humans use wood because of its excellent structural characteristics as a building material to improve our physical lives. It is also aesthetically appealing.
When wood was part of a living tree it had a fairly stable moisture content summer through winter and was protected by its bark, oils and other constituents. But when harvested and used as bare lumber, particularly outdoors, these protective features are gone and lumber, left unprotected, has a short remaining physical existence. Sunlight exposure (Ultra Violet Energy Degradation) attacks its physical surface integrity. Microbiological organisms use it as a residence and food source. Wet/Dry cycles of rain and standing water, then sun cause wood to first swell, then shrink in a process of dimensional instability that causes cracking, twisting and the ability of water and biological attackers to easily get deeper into the lumber. Left unprotected, this process reduces wood's physical strength, substance and aesthetic.
Sealing wood when dry, particularly with oils, incorporates into exterior wood's near surface layers ingredients that satisfy it's otherwise mechanical desire to absorb water, thereby curbing wet/dry dimensional stress cycles and reducing wood's hospitality to organic decay mechanisms. By moisturizing with oils, sealing also allows the wood to be less effected by UV energy stresses. This process results in protecting wood, in part, from the physical attacks that it would otherwise suffer, allowing us to extend its useful life outdoors. Sealing buffers its physical deterioration and can actually enhance its appearance!
Should I seal or should I paint or stain?
This is sometimes a matter of personal choice but these facts should be considered.
Paint locks in moisture
and can encourage rot in damp climates, particularly on decks. Additionally,
it cracks, chips and peels, quickly on horizontal surfaces such as decks.
Like heavily pigmented stains, paint obscures or totally hides the vision
of wood, showing traffic patterns on walking surfaces. While paints and stains
usually require less frequent re-application, they are more labor intensive when that time comes. While on vertical
surfaces paints are usually the way to go for aesthetic and durability
reasons, for those that love the natural look of wood, sealers and transparent finishes are still sometimes the way to go.
On Decks and Fences penetrating sealers are simply hard to beat. See "Why use oils", for an explanation of why we don't make water based systems for wood! Particularly, on Decks and Fences,
we recommend the oil-based sealant approach.