Shields All is water resistant not water proof. What i did on my background is grouted > painted > several coats of shields all > then you should put silicone over areas that will be constantly wet. I like to keep my vivs wet so i go ahead and just cover the whole thing in silicone. If you are sealing with Shields All correctly water shouldn't really be getting through the sealer into the Great Stuff. How many coats of Shields did you use? I usually do 3-4






hope that helps
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