Chemically strip the varnish, specially good with the citrus based strippers, won't touch the epoxy but will easily strip the varnish.

Also won't expose the glass which can easily happen if you sand too far.

Bill H.

oh yeah, very carefully mask any part of the deck you don't want the varnish removed. Poly sheeting is very resistant to the stripper chemicals. Masking tape isn't real resistant, packing tape is.