Which FET package are you trying to mount on SK104-STCB? There should be better ways to connect the drain terminal.
I doubt that the solder pins are intended as electrical connection. Neither the heatsink surface, particularly with a clip mounted transistor. Even a raw aluminium surface won't give reliable contact if the transistor isn't screwed to the heatsink.
Milling would be the most appropriate method to remove the coating, grinding should work as well.