Was this walnut media Lyman TuffNut by any chance? It is charged with rouge, and they use way too much of it. If you mix that TuffNut about half-n-half with plain walnut or corncob media, it's cheaper to use and it gets rid of the mess.
Adding NuFinish probably made it worse.
"The only way to stop a bad guy with a gun is with a good guy with a gun"