It may depend a bit how thoroughly the hull was stripped before the epoxy was put on. I know its not quite the same but I had one boat that had been coated in resin and glass but the original varnish had not been totally removed so the sheathing just pulled away in large patches. Polyester resin isn't a glue like epoxy but keep your fingers crossed that there is some old varnish left to seal the timber and try a heat gun. Walnut shells might work, maybe some one has used them on epoxy and can advise.
Epoxy responds to abrasives quite well. If you try a sander I would suggest using a spong drum sander in a drill as they don't clog up and cut with the grain; find one here http://www.axminster.co.uk/black-and-de ... rod481332/
They take a bit of getting used to. keep it moving smoothly along the planks, keeping it always level on the wood. etc. otherwise it will eat the wood. With care they will get in quite close to the corners and scrapers should do the rest. Not a very quick solution but I think quicker than a belt or orbital sander. It will throw the dust every where however.
Just as a matter of interest is the inside epoxied as well? if not everything John said will certainly be true.