Do not use sand paper for the same reason as the brown "cookie" style Scotch bright disks. You want to avoid abrasives in the engine at all costs. A good scraper, not a razor blade will get 99% of it off. You can use a wire wheel or one of the disks I mentioned earlier to get the rest. There are chemicals that also will dissolve old gaskets and gasket cement, I don't
recommend them but they are available.
A good scraper has a nice clean 90* edge, not a sharp edge like a razor.
Artificial Intelligence is no match for Natural Stupidity