Hi Paul,

: I would appreciate your thoughts before I go about this. I am thinking about
: sanding the scratches out of the paint, which in some cases may mean going
: down to the epoxy undercoat.

This is fine, if you go through the epoxy to the glass you will need to apply another layer of epoxy before painting.

: Do I need to remove all of the paint and redo
: it, or will new paint stick to old paint?

The new paint will stick to the old paint. You will need to scuff sand the old paint (remove the shine) to give the new paint a mechanical bond.

It is, in fact, better to leave the old paint because it is a good primer. This is assuming that the old paint has good adhesion to the epoxy, if not you should strip it off and redo from the epoxy up.

Ian