This homeowner caught this deck just before it was too late. Previous painter put some sort of latex on it, which is a mistake for decks. The professional choice is solid stain. There are multiple brands, but the best I have found is PPG. Sherwin WIlliams has one but it’s been reformulated and is rather thin. Flood is a little better, with better coverage and durability. But the PPG is superior, no doubt.
Here is a pic while I was in the first coat. The wood is old, but it would have cost the client three times more for new wood. With some minor supports that I put under the deck, some tightening of screws, and the PPG solid stain, they are back on track with a long lasting deck.