Is it an alloy headset or steel
If it's alloy it could be hard anno like on Pro class III rims well hard to remove , try caustic rather than Oven pride
If it's a steel headset it could be a type of black chrome plate ,,, in which its take it to a chromers for stripping
Failing these ,,, have you tried sanding it not too hard
![Undecided :-\](http://www.radbmx.co.uk/forum/Smileys/smilies/undecided.gif)
or big chisels and angle grinding
![Cheesy :D](http://www.radbmx.co.uk/forum/Smileys/smilies/cheesy.gif)