Whenever you cut into a piece of pressure treated wood, you should treat the cut end with a chemical called End Cut Treatment. This insures the full anti-rot protection of the piece of wood as it came from the factory.
Maximum preservative value will be obtained by applying end cut treatment liberally to dry wood, let it dry and apply a second coat. Good preservative value can be obtained with wet wood if you let the cut surface get dry to the touch and then apply the end cut, you still get pretty good absorption of the preservative. If you really must cut dripping wood and treat it immediately because of the demands of the job, applying end cut to dripping wet lumber is better than not applying it at all. Whenever you cannot apply two coats, such as rapid assembly of butt joints, apply as much treatment as it will absorb, wet or dry.
Logic would say that dripping wet wood would not let any end cut treatment get into the lumber. Think of a glass of water. Put a drop of oil in the glass, and it just sits on the top, no penetration. Put a drop of strong coffee into the glass and the entire glass of water becomes darker. End Cut treatment falls in-between these two extremes, it is fairly soluble in water, and to some extent will flow through the water into the lumber. So when the lumber finally dries out, there will be some preservative chemical in the wood fibers.
Remember that the new colour matched end cut treatments are not as powerful as the old bright green stuff. Any lumber ends in ground contact should be treated with the full strength end cut treatment, but by definition they are hidden underground anyway.
**Originally published as an article by Jon Eakes in Home Builder Magazine, the magazine of the Canadian Home Builder's Association.
For information on the changing nature of pressure treated wood, follow this link.