After completing my sign project, do I need to apply a varnish over the FolkArt Sign Shop Erasable Liquid Chalk to make it permanent?

No, once fully dry, the paint has a no-smudge beautiful chalk effect.  As a matter of fact, if the message or design were sealed with a spray clear acrylic sealer, it would no longer be erasable and the surface could no longer be reused.  However, if the project is to be a permanent, non-changeable sign, then yes, of course, the surface can be lightly spray sealed with a clear acrylic sealer to seal and protect the design.