Without the proper surface and substrate preparation, staining wood is a waste of time and money. Whether refinishing the exterior of a home or a deck, stain appearance and lifetime depend on correct wood preparation. This is one factor that separates professional wood restoration companies from handymen, painters, and “do it yourself” homeowners.
Particular treatments depend on the condition, age, and species of wood, as well as if an old stain is still on or in the wood. If no stain or old, weathered stain is present, a specialty wood cleaner is used to remove surface and substrate dirt, mold / mildew, and remaining stain. My recommendation is a solution of 1.5 – 3% Sodium Hypochlorite (pool shock is 10%) or Sodium Percarbonate (Wolman’s Deckbrite). Pump spray 8oz per gallon of water. Dwell 15 (?) minutes.
A low to high pressure wash depending on the wood is used to remove the wood cleaner and dead wood/organisms. Neutralizer / brightener is next applied to all wood to neutralize the caustic treatment and insure the wood is at a proper pH prior to staining. Citric acid is the most highly recommended neutralizer, although oxalic acid is also popular.