 Then you can use the Olympic Deck Cleaner. Spray it on. I think it sits on for about five to ten minutes, and then you rinse it off. And that's probably the best you can do in terms of prepping that surface for stain. Now, you used the word seal, talking about sealing a deck. Sealing a deck is not only going to do much for you. What I would recommend you do is stain the deck. And, of course, you have choices on stain. They come as transparent, semi-transparent, and solid color. And I'm a big fan of solid color because the more color you put on there, the longer it lasts.