Sand or scuff it with a scotch brite pad, either way to get the surface as smooth as needed.
I thought you said it was top coated already, so if you don't sand thru it exposing the primer I can't see why another coat of primer is needed unless the previous top coat is not compatible with the urethane you're going to use.
One thing to remember is too much paint film lends to paint cracking.
Dan
