Yes, it is possible, but you need to be well prepared. After applying the final coat of epoxy, wait for a week for it to fully cure, even if it is longer than what the manufacturer recommends. All types of marine epoxy produce a waxy residue called "blush" during their curing process, which can prevent a subsequent coating from properly drying. Patience is key here.

Before sanding, clean the epoxy with hot soapy water and a sponge, then rinse thoroughly and let dry. Sand the surface of the epoxy with 220 grit sandpaper and make sure to sand after washing to avoid spreading the waxy blush, if present, as this can be very difficult to remove.

For varnishing, the first few coats do not need to be thinned as the epoxy prevents the wood from absorbing the thinned varnish. Keep in mind that epoxy offers a very limited protection against UV, which fades over time, so it is important to apply a minimum of six unthinned coats of varnish to protect the epoxy and the wood.