If anything we could assume coat's lamination was intended to mimic leather and MAYBE to offer some water-resistance, not waterproofing. An excerpt from one of Renee's interviews "it’s laminated cotton that we painted ... Leather would have become wet and very heavy in that environment...etc". The fact that she rules out the use of leather could indicate that it was the aesthetic goal. Her solution to working with a coat, that was knowingly to be soaked, was opting for cotton, as leather would have become too heavy during Ryan's fight scene. I would steer clear of spray products like "Rust-Oleum Neverwet Fabric Water Replant (clear formula)" or even "Crep Protect" intended for clothing because they aren't permanent solutions. You'll find yourself having to reapply the stuff every couple days and they wont provide that extra shimmer.
my 2 cents. The film crew painted over lamination,whatever that means, I wonder if the integrity of the coat's color was only meant to last the length of production. I've seen on this thread, those of you who know WAY more than us laymen, suggest that painting over varnish (our best substitute for lamination, and solution to weatherproofing) wouldn't provide lasting results. And if we were to apply paint then varnish (as practically intended), the dilemma becomes, how do we expect to recreate the same look as the film crew if we must reverse their production method. It seems to me altering the coat is a matter of choosing to recreate the coat's shimmered finish or choosing a utilitarian weatherproofing. Anyone who can kill these two birds with one stone (and I hope you're out there) please share.