Some years back we worked on a pine deck in Titirangi West Auckland. The deck was only 10 years old however the decking was installed incorrectly for a damp shaded environment with small gaps between the boards, in this instance hardwood decking should have been installed with large gaps for ventilation. The pine decking had been water blasted so many times that the deck was left with a fur type surface that was permanently slimy and slippery. Repetitive powerful water blasting should be avoided as it breaks down molecular structure of the timber. We installed hardwood decking (more resistant to mould) instead of pine, with larger gaps between the boards (for ventilation) and recommended a yearly oil based treatment to inhibit mould growth. I visited this deck recently whilst we worked on an another project nearby and after 7 years and it sill looks great!