Firstly consider what coatings (if any) you have on your timber already:
If you have new (bare) timber, firstly allow the timber to weather for 4-6 weeks.
For previously painted or varnished timber remove back to bare timber using a paint stripper, or sanding.
For previously stained and oiled timber, clean with Cabot's Deck Clean as detailed in the Clean section of this How To Guide.
For best results on badly weathered and grey timber, sand timber back to fresh substrate, then clean with Cabot's Deck Clean as detailed in the Clean section of this How To Guide.
Note: Fill holes and imperfections with a suitable timber putty.