If the plugs threads stick out into the combustion chamber that's not a good thing as hard carbon deposits can tear up the heads threads when you take the plug out for what ever reason you need to take the plug out for. I also if I think of it I'll power wire brush the new plugs threads and a little dab of never seize on the threads and everytime I round trip a plug and have access to a wire wheel and never seize I do this. To clean the plug hole a little spray of carb cleaner but I don't do this unless it looks really nasty.What about the nasty that goes into the cylinder you ask. Well I hope it goes out with the exhaust on the first time it fires as aluminum powder that fine in the never seize will burn and burn clean up and out the pipe it goes. what do yo think is in the good old firecrackers aluminum powder and potassium chlorate and maybe a pinch of flowers of sulfur.
Norman