Depends on the condition of the existing FG and how much effort you would like to put into the project.  I'm going to assume you are just attempting to smooth the surface for aestetics.

The easiest way, if the FG is still sound and bonded, would be to scuff the existing skin and add fill coats of epoxy (thickened with phenolic micro balloons if painting) then sand to smooth/fair and finish with a UV protective coating of your choice (varnish/clear/paint).  You don't really want to sand the existing FG too much to avoid damaging the cloth fibers, although I'm guessing the FG is entirely for water proofing rather than strength.

If the existing FG is damaged or delaminated then you need to at least grind these areas out before commencing with the above procedure.

The glass is in good shape.   It was put on to repair a rivet that pulled the bottom and to protect from rock damage on New England shores.  The boat was made from fir marine grade plywood and air dried white oak.  The white oak ribs were held onto the plywood with copper rivets