What do you do when over-plasticized dope doesn't harden enough to sand well? Simple, reskin the wing.
These are photos of the original molded Mustang, the blue E2-S. This airplane was the first of a series of airplanes later called "Snaggletooth". I screw up a lot but I'm very good at fixing mistakes. This first Snaggletooth had the fuselage lengthened, the wing cuffs replaced and the wing reskinned before it flew for the first time. It was a wonder that it flew well enough to place 3rd at the NATs in 1974. Later versions benefited from the lessons learned. Ah, well.
Al