Wood Preservation
How wood is treated
Industrial preservation uses pressure processes - vacuum/pressure cycles that force preservative into the wood - to meet retention (amount of chemical per volume of wood) and penetration standards set by the AWPA use categories (UC1–UC5, from interior dry use to ground contact and fresh water). Treatability depends on species, sapwood versus heartwood (sapwood takes treatment far better), moisture content, and incising to open the surface of refractory species.
Preservative families
Waterborne systems include CCA (restricted for most residential uses since 2003), ACQ, copper azole, and borates (interior, protected applications only because borates leach). Oilborne systems include creosote and pentachlorophenol - both restricted-use and requiring certified applicators - plus copper naphthenate for field cuts and remedial work.
Field work
Pressure-treated lumber is not automatically protected after it is cut, bored, or notched; field-cut ends and exposed surfaces in ground contact or high-decay hazard should be brush-treated with a labeled preservative. Do not use treated wood where the label or standard prohibits contact with food, drinking water, or sensitive uses.
Traps
Assuming all "treated" wood is equal regardless of use category; treating heartwood and expecting penetration; using borate-treated wood in soil contact; forgetting that creosote and pentachlorophenol are restricted-use; overlooking the need to re-treat field cuts.
Rule of thumb: match the use category to the exposure (ground contact needs a higher retention than above-ground decking), and treat every field cut.