Using a standard rubber sucker attached to a variable speed and reversible-drive drill. Normal procedure; applying coarse then fine grinding paste.
This is an inlet valve. Note the blackened face of the adjacent exhaust seat suggesting that it had not been closing properly. The plug hole is packed with tissue to keep out debris and paint.
Also note the marks on the head face inside the cylinder outline; it would appear that something serious has happened at some time in the past.