I shot the matte sealer coat on the test piece, and it did indeed "waterproof" it for normal purposes. So I shot the trunk as well, and it cured nicely to a durable looking finish.

I then read the can of the GM spatter, and it did not say that topcoating was necessary. So I shot some on the back side of my test piece (without bedliner) to see if the bedliner contributed to the solubility.

Nope, even directly on the substrate and fully dry, the spatter wipes off easily with water. Sealant is indeed necessary.