The DC4 should only be touching the bit of the blade that you are going to be grinding and polishing away, ie. the bevel. If there is any sort of film coating on the bevel that you do not want to clog your stone, why not just sand it off? Say, 400-800 grit wet and dry paper. That will leave any film elsewhere to wear off in its own time, and provide some extra protection before then. Any film elsewhere on the blade will be no worse than one of those black coated blades, and those don't cause particular problems for sharpening. The other thing, use your DC4 wet, if there are particles of film that are removed, the water will help to keep the stone clean.