We just completed a redwood deck, using Thompson's higher grade water seal and Cabot's semi-transparent redwood stain. After only two weeks we are starting to see dark areas that almost look like mildew, but may not be. Any ideas that my help would be appreciated.

Actually, you really should have only done one or the other, that is either water seal or use the Cabot, not both, with the Cabot being the much better option. Once you but the Thompson down, it formed a wax-based repellent on your deck, and therefore the Cabot would have a hard time penetrating into the wood. My guess is... the Cabot Stain (which is an awesome product btw) did not penetrate well in these dark areas and is actually sitting on top of the wood (did not penetrate). The Cabot already has the "water sealing" properties you were seeking, plus the all important UV protection from the sun. Are these spot also a bit "shiny?"

Thank you so much for your response. No, the spots aren't shiny. It's really turning dark where the knots are, and other places too. It is not uniform. Since we relied on our deck builders and didn't do enough research, what do you suggest we do now?

Are those areas tacky... a bit sticky at all? Since the knots are the hardest part of the wood, it does not surprise me that the knots are the darkest. If the stain did not penetrate, and sat on surface, it would remain tacky for quite a while and collect dirt and dust. Did the dark areas gradually appear?

No, they don't feel tacky. The dark areas are appearing gradually.

Unfortunately, if they are no longer tacky, there is no quick fix. Sometimes you can use mineral spirits on the spots, but once dry, this is no longer an option. I would suggest you call Cabot at 1800-US-STAIN. They have pretty good techs who may be able to suggest something. Ask for Matt McKay, he is the best one there. I kind of think your deck builder messed this one up. You can strip it and start over, or just live with it and hope the spots dissipate after a while. Whatever you do, DO NOT add any more coats of stain in an attempt to level out the color. This will just make the situation worse. My fear is that these spots will eventually peel. Sorry this has happened to you... and good luck!

Thanks so much for your advice. We will call Cabot and go from there. Aloha