Waterproof stitching?

[Scythe13]

Hi everyone.

As you all know, I'm a fan of modifying stuff.

One thing I'm looking at is waterproof stitching. If I want to stitch something to the outside, or underside of a snowboarding waterproof jacket, how can I make sure water doesn't come in through the stitching holes?

The things I've looked at are all inflexible. I am after something SUPER flexible, as in, able to move in completely the same say as if the jacket was unmodified, and also so that I can scrunch it up into a ball and put it into my bag (I'm not on about putting it in wet, incase anyone wants to say how bad that is to do).

Thank you all in advance for your ideas.