How do I tell when I've sanded the varnish off and I'm down to the epoxy?
For me, it's the feel. When wet sanding with 400 grit paper, varnish feels soapy. Once I get through the varnish and it's just epoxy, the soapy feel goes away. Also, if the varnish is relatively new, there's a change in the smell, but that's pretty subtle and not always there.
Good luck,
Laszlo