Each of the tools mentioned
-- from the fingernail to the cabinet scraper -- has it's place in the scheme of things, depending on the variables mentioned: hardness and smoothness of the surface; composition of the offending material; patience of the practitioner. Before I spent much time with a razor blade -- or a fresh mat-knife blade, maybe in a little wood handle slotted on the band saw ? -- I'd make a new tool:
Take a firm new putty knife (or a springy one if you like that idea better) and carefully create a square "blunt" edge on the end of blade. Perhaps you would hone this square end on a stone, making as crisp an edge as possible.
With this tool you would slowly and steadily push the paint off the surface, assuming the substance has some flexibility, by starting at one edge of the paint. Warming the paint with a hair dryer would probably make things considerably easier.
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