Another 10 to 20 swipes on the abrasive paper and inspect the progress. The blacked areas are the guide coat still present in low spots between the areas polished by the abrasive paper.
Another 20 or so swipes and note how the shiny portion of the gasket area is increasing in size.
Another 20 swipes
And the final 20 swipes produce a gasket seal area without any evidence of guide coat left
The head material removed during the process. The slurry produced from this process is also highly abrasive. You may want to keep the slurry away from any nearby hand tools or power tools.
The final step is to gently wash away any silicon carbide material from the glass pane. Rinse the pane prior to wiping it down so as not to rub any abrasive into the glass surface.
Carefully store the glass pane for future use.