I have never used it outside, I did use some 18 months ago after another fungus gnat outbreak. I put a quarter inch all over the top of my pots.
It sucked all the water from the soil underneath, and turned into a soggy goo that stuck to anything that touched it.
After a week the top started to go Brown and it looked like a giant lemon meringue pie, with a weed plant sticking out the middle of it lmao.
I have read it's good to put in soil as it holds a lot of water, and slowly releases it. As well as deter pests when dry.