how clean does the gasket suface have to be and how do you clean it properly
Depends what your using oring or gasket. best way to clean is with a new razor blade and PATIENCE. you have to get all the old gasket off, without gouging the cylinder. you can use a piece of fine scotchbrite and brake cleaner by hand but never with any power tools. and only work around the bore never at or away from it. lomg smooth motions so notto create a low spot.
i don't know if it this best , but after cleaning old gasket with a blade and solvent. i use a thick window glass piece and a 1000 grid sandpaper. i fix the sandpaper to the glass ans resurface a little to get it clean near perfect.
I've used a knife sharpening stone to not round the surfaces, but usually a razor
blade and contact cleaner
Berrymans B-12 chemtool and your thumbnail.
The Berrymans will take off your paint so be careful.