Albert Bacci of the Noble House Coatings company in Toronto came to the TV studio to share his experience in applying outdoor stains.
First rule, if there is mill glaze (that washer board look caused by the planing blades in the lumber mill) on the boards, you must sand them off or no stain will penetrate properly. Albert recommends 80 grit sandpaper, most easily a belt sander. Don't sand to a furniture grade smoothness as this in itself will prevent stain from penetrating.
Second rule, if the wood has gone grey, use a chemical "wood restorer" that will remove all the dead wood.
Opaque stains are not abrasive resistant and cannot withstand walking, so they are only used for vertical surfaces like fences or wood siding.
With the proper preparation, most quality stains will give durable protection to the wood.