Since I have had 40 years experience with this problem....I will give
you my perspective:
In the Zam, or any other polishing compound, you will find a
petroleum product...it is mixed with the grinding particles. During
the process of running the "mop" or the polishing pad which has had
the polishing compound added to it....the heat will start to remove
the outer layer of metal...silver in this case that I mention.
So..the layer of metal becomes black in the polishing compound. The
black is the metal wearing away and removing fire scale if that is
the end result, or a brilliant polish.
The metal WON'T polish under the buildup of the black "crud". I keep
a clean (to start with!!!!!) soft cloth at the polisher and when
there starts to be a build up, I wipe the black stuff off...it is
never ending, but by doing this you will finally get a nice polish.
As I repeat, I have had many years of experience with this dilemma.
HOWEVER, you may be overloading the polish pad with too much
compound....notice you are keeping it raked, tho. Give the soft cloth
a whirl to see if that solves the problem.
Hope this helps.
Rose Marie Christison