We have a concrete patio under a huge oak tree... When it rains, the water doesn't dry quickly, and a thin green film appears every year...
We power wash it back to concrete white, and start all over. I'd like to seal it - to help prevent the green grundge. Most articles on the Internet frown on Thompson Water Seal, as it creates a sealed layer that can't "breathe" the moisture from below.
SO - what else do I look for and purchase in the big box stores ??