Topical/surface treatments for wood usually limit protection to the surface area as it is applied by brushing, spraying or dipping. Although regularly coating a surface with a paint or sealer may help protect wood from the elements, it won’t necessarily prevent it from rotting or being attacked by insects.

Pressure-impregnated - Preservatives are infused into the wood, beyond just the surface. Pressure Treatment (PT) is the general term to describe the process for infusing/impregnating the wood fibers with preservative chemicals, removing any excesses, and leaving behind only enough chemical in the wood fibers (retention) to protect the treated wood. The AWPA sets appropriate chemical retentions depending on their intended use/requirements, based on performance data derived from long-term scientific tests. The AWPA wood preserving standards are reviewed by their technical committees every five years to ensure that retention levels are appropriate and that a given preservative formulation is performing as expected.

There are three categories of pressure treatments available for wood:

1. Waterborne treated lumber is generally used in building structures that are residential, commercial and industrial.

2. Creosote-treated lumber is mostly used for treating guardrail posts, railroad ties and timbers used in marine structures.

3. Oil-borne treated lumber is used when treating utility poles and cross arms.