I have PLA and PETG filament.

I hear that 3D filament absorbs water and causes problems when printing but after printing they can be used with water and they are water proof.

So my question is why is it different after printing/what has changed to make it now waterproof?

* Is PETG waterproof or does it absorb water?
* Is there a limit on how much water PETG can absorb or will it keep going until it splits and turns to mush?