In theory, applying a coat of Future (or any other acrylic clear coat) over enamels, then using an enamel wash should have no problems. The trick is getting a good, solid covering of the clear coat. If you don't get the clear coat into all of the knooks-and-crannies and then apply the enamel wash, the wash will eat away at the base. It's happened to me a few times. I really suggest using either an acrylic base and enamel wash, or enamel base and acrylic wash. It seems to me from reading about others methods, that most use acrylic bases and oil (enamel) washes.
Jesse