Need more than 50 strokes IIRC - you need to get it showing all the copper, all I can see is a piece of copper showing on the left of that picture.

Get a big piece of glass and carry on sanding, i'd say 2 minutes sanding one way, rotate 90 degrees, sand 2 minutes etc etc.

You need to get it so its all copper, then move up the grit ranges

my input, someone correct me if im wrong!