Have your tried a sharp, stiff scraper? Something with little flex as you really want be able to knock the remaining material off. A sander would likely get plugged up in a hurry....
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)