The cleaning tool I was referring to is a small blasting unit that is made
specifically for abrasive cleaning spark plugs. Only the threads are
exposed to the material and the electrode area of the plug. I use 3/4 inch
masking tape to cover and protect the threads if they don't need cleaning.
The tool is sold for $30.00 with extra abrasive for $5.00, through Eastwood
(www.eastwoodcompany.com) or J.C. Whitney although I don't know if J.C.
Whitney has extra media. Or, if your lucky enough to have a blasting
cabinet then there is an adapter for $14.00 that will do what Miles is
speaking of. I like aluminum oxide as a cleaning media for all purpose use
and garnet for aggressive blasting. The aluminum oxide just lasts longer
than the rest, and like you were saying it, as well as crushed glass etches
the surface, but then so can some dish soap. The cleaned plug is better
than a dirty one and will do the job.
A popular function of the tool is it has two settings, Blast or just air.
If all you need to do is dry it out it's been a handy tool. Also it is
quite a bit easier than starting up your blasting cabinet, dust-collector
just to clean one plug.
As to NGK plugs over other plugs, I have just as good performance with
Champion or Denso plugs as I do with NGK. With Champion plugs the top caps
may not unscrew, but if you already changed out your plug wires to silicone
from the wire there shouldn't be any problem.
Just a note on plug threads, use a quality anti-seize like "Thread-EEZ" and
the threads in the cylinder head will last a long time.
Bill