The GC instructions tell you not to use an S.O.S. pad (steel wool impregnated with soap), but I've never had a problem as long as the soap is fully rinsed away before soldering.

Hallo Joseph,

steel wool makes trouble, if the soap contains chlorides! Then, copper surface reacts with these chlorides. You can see it: The colour of copper changes to orange a bit. But what is much more annyoing, chemically tinning will not work properly...

I have made very very good results with a tool from 'SENO' called 'Polibloc'. Gives perfect clean surfaces, also good when chemically tinning the board. After polishing the surface I remove the dust with a bit destilated water and a piece of soft toilet-paper. No touching the surface with fingers, before tinning!

Kai