Almost NO clearcaot will work over the enamal metalic paint (I say almost there may be something I don't know)
That includes silver, gold, copper or any other metalic paint by Krylon, Wal-Mart, K-Mart, etc. Not even their own matching brands!
Don't do it!
Clearcoats seem to turn the metalic paints "blotchy".
Future Floor finish won't do that. but...be careful with it...too much over white will turn yellow. A little Future goes a long way to protect the finish from smudges and fingerprints.
The ONLY exception is automotive matelic paint! They are designed for clearcoats but, stick with thier specific brands!
I don't have all the answers about what WILL work. Over the years though, I have learned a lot about what WON'T work.
sandman