Try using NAPA Flowable Silicone Windshielf and Glass sealer. Assuming you have access to to a NAPA auto parts store or their web site. Be sure to get the clear, not the black stuff.
There have been a gazillion posts here about how well this stuff seals silnylon.
However, I’ve found that it will adhere also to some PU coatings. It would probably not be an effective bonding agent, but if it adheres to the PU, it should work as a sealer if rubbed in, as Roger says. Note that we are having high temps in much of the US now, so you may have to put it into a room with AC to get it to dry.
Still another possibility, when no amount of planning will allow all the exposed seams to be on either PU coated fabric or SIL coated fabric: Just coat seams on the PU coated surfaces with SeamGrip, and on the SIL coated surfaces with your favorite sil sealer. In addition to the NAPA product and the Sil-Net, there is a Sea to Summit Product called Ultra Seam Sil that flows better. But they all cure in the tube eventually, so it makes no sense to stock up on any of them.
Good Luck.
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