It's not a good idea for several reasons.

First, the varnish is the UV protection for the epoxy. Putting the epoxy on top of the varnish is like sitting on top of a beach umbrella. The shade is under you where it does no good, not above you where you need it.

Second, because of the chemical compositions, even if you sand it epoxy makes a poor bond to varnish.

Third, even if it did bond perfectly, varnish is mechanically weaker than epoxy. It's much softer and resists shear forces less. So the varnish would be a very weak glue between the epoxy and wood. The hard epoxy layer would eventually pop off.

Always remove everything from on top of the old epoxy layer before adding a new one, and sand the old layer to roughen it up and increase the bonding area. If there is no old epoxy layer, sand down to clean wood - no varnish, paint, oil, etc.

Good luck,

Laszlo

OK. How do you know when you have completely removed the varnish, but no more than that?

I haven't needed to do this yet, but the day will come.   

With the Schooner varnish that CLC sells, it's pretty easy. Lightly wet sand with #400 paper. As long as it's varnish that you're sanding, it'll feel soapy. When the soapy feel goes away, you've removed the varnish and are into the epoxy. The smell changes, too, but I don't really have a good description for that.

I have only used Schooner varnish over epoxy, so I don't know how other brands behave.

Laszlo

OK,I'll remember that.

But #400...that might be a long time sanding. I assume wet sanding, right?  

Nick Shade has a video on removing old varnish. He used a paint stripper.  See it here -    https://youtu.be/Jig_pGN3Yus.