[header] How to seal and stain pressure treated wood decking [title] Remove staining, mildew and old finish with a commercial deck cleaning solution. [step] Pressure washing is almost always a must on older decks, but be sure not to use too much pressure. You can damage the wood if it's turned up too high. 
Usually 1500-2500 psi is adequate for pressure treated lumber. Stay between 1200-1500 psi for cedar and redwood.