Friend has this quarter that they have already done an acetone bath and no results. Any ideas what the next step is or is it a loss in collectability? Here's his pics.... (Also, don't know what the gunk is either).
To answer your question, no, I don’t think it can be saved. In 1964, 90% silver planchets were routinely washed before being fed into the stamping press, cleaned with detergents and a borax solution, then rinsed. ANY remaining residue remaining after the final rinse eventually produced silver chloride, a product of the cleaning agents being trapped under the surface of the silver, and over time produced milk spots or blotches. This 1964 Washington appears to have missed a final rinse. Franklin Halves are notoriously known for milk spot problems as well. Professional conservation may be able to reverse this malady, but I don’t have any knowledge or experience in this type of rescue action.
