Just bought a T6, and then bought a pair of VW roof rails and just been sussing them out. The fixing kit contains 6 self adhesive neoprene washers which I conclude would provide the seal but I would be inclined to seal the bolt threads as well and loctite 577 springs to mind. You use it to seal metal to metal threads in plumbing & heating applications which often would be under 1 - 2 bar pressure so should stop rainwater. I agree it all looks a bit of an afterthought with the plastic spacers to fill the channel. Any comments appreciated (I'm a novice T6 owner and forum user!)