Ken,
I agree, never sand until the surface has been cleaned. I learned that years ago on big cars. I use no silicone or any kind of mold release any my mold process except for the pouring of the inner plug so it won't stick to the already poured outer mold. I put my pattern in the outer mold then paint the areas where the mold rubber will touch with the 2 part release, When it's all done, I wash both parts of the mold in hot water with soap!
I go thru a long process cleaning the casting before I paint. I can and have used alcohol, but it isn't part of my regular process. Maybe it should be. Maybe silicone in the actual rubber is creating the problem somehow.
It's a continual learning process and it I can live another 100 years I might have all the answers!!!!