The presence of an existing colored stain or sealer. Depending on the previous product used, and the application of that product, it may or may not wash off 100%. Some products wash off very well, while others are very hard to remove.
We always use a very high quality (non-bleach based) cleaner/stripper, but this will not remove all of a previous stain, and it is not a good idea to try to “blast” this residue off with high pressure.)
Marks from flower pots, grease spots from grills, fixtures mounted to the deck, sap, and previous stain residues may not be removed and may be visible after cleaning or staining.
Some pine decks may have green coloring on the board from the factory pressure treating process. In these cases, we generally recommend cleaning the deck, assessing the amount of residue, then re-staining the deck with the same previous product, or a very similar product, or even a solid color stain in the worst situations.