I very carefully scrape with the round end of a hacksaw blade to get the heavy stuff of if there is any. Then wire cup brush on a die grinder. be careful not to put scratches in the head with the hacksaw blade as that will create spots for carbon to adhere to and stay off the gasket surfaces with the brush.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!