Most of the citrus strippers will work for this, they are slow to eat varnish but usually won't harm the epoxy.

The trick with any stripper is to keep the surface wet, if working outside laying a layer of paper towels onto the wet surface will help to keep the stripper from evaporating as quickly. Take your time, let the stripper work, just watch it doesn't dry because it's harder to remove after it's dried out than it was before it was applied.

Bill H.