The waterproofing is a separate step. I normally just use clear spray paint, the exact brand I use is in one of the pictures.
You should not be using fiberglass AND Bondo in a build, one or the other will work fine and be lighter and cheaper. The fiberglass route is not something for a new maker, as if you do it wrong, you wind up with a bunch of bubbles and gaps in your armor, which make it super fragile