Read Tuts here on-site or even youtube DIY vids to understand how to use and what needed additional materials - brushes, mixing cups...etc.
The resin does soak into the cardstock but it begins to harden quickly, long before the added weight of the liquid could soggy the cardstock and deform it. Remember, you are creating a composite material -- combining two materials to make something that is stronger that the two individually.
As for scaling the armor, stay tuned to the Pep Scaling Thread. We are trying to gather everyone's build measurements to compile some accurate size tables.