[Iscool]

To be full of grace (or full of anything), a person or object cannot contain any amount of another substance, else it is by definition not full of grace (or whatever the substance in question is).

Grace is not a substance...

[Wyrd bið ful aræd]

Way to jump on the main point.

Of course grace is not a substance. The point is that if one is entirely full of grace, there is no room for sin.