I have been cleaning the spark plugs in my model T with a spark plug blaster from Harbor Freight for years. I had a friend tell me that I should not do that, because I was destroying the porcelain on the plugs. What are the thought's on this matter?

I would tend to agree with your friend if you are using older type plugs like Champion X's. What plugs are you using?

Use a blast media that is softer than the porcelain.

Several media are available. I would agree with Marc that a medium softer than porcelain should be OK.

Walnut shells???

Aluminum oxide would be a good choice.

John, I am running Champion X's take apart plugs.
I use the media that comes for the plug blaster from Harbor Freight.
This new gas does not burn clean. My friend says that I should clean the plugs with brake cleaner.

If the plugs are the take a part kind, a buffing wheel and compound cleans them nicely.

I clean 'em out with good ol lacquer thinner. Seems to work great, doesn't harm anything except dry your fingers out a little.

The media from Harbor Freight is probably carbide and too hard. I use glass bead media which is softer than ceramic and is used on aircraft aluminum parts such as a carburetor or fuel pump and parts that cannot be cleaned with de-greaser that contains phosphoric acid.

The reason that old stock spark plugs work well is because the porcelain has a glaze on them, versus modern plugs with a porous, or bisque, finish. The glaze I'm referring to is on the lower part that extends into the combustion chamber, not the upper part outside the engine which is always glazed. The glaze prevents saturation from carbon, which is conductive and can short the plug. Sandblasting can remove the glaze and uncover the porous material beneath. It's not an issue with modern engines which burn far more efficiently and don't produce near the fouling material that our antique engines do.

Media are plural.
The singular form is medium.

Steve,

My singular form is XXL, sorry to say.

I clean Champion X plugs with red Scotchbrite and some MEK. Very seldom do I need to do this, plugs seem to last forever once cleaned and gapped. If your car is fouling plugs in less than tens of thousands of miles there is a problem that you need to fix.

Jerry hit the nail on the head, you really shouldn't us any blast media as you will surely remove the glaze on the porcelain material. They will still work, but foul easily. To preserve the plugs on assembly you should find some old asbestos to replace the original in the copper sealing rings. You can still find small amounts and by mixing it with water, bring it back to life for this job. BTW, I have some if you need it. Using the new style copper crush washer generally result in broken plugs and really don't seal well. All of your rebuilt plugs should be put in a spark plug tester to be sure they fire consistently and then run the pressure up to see where they quit. You'll be surprised how many don't fire or don't fire consistently at just 70psi.

Here is a picture of how you can take a piece of old asbestos and wet it to roll it in your fingers to almost a putty to lay into the copper ring. Don't over tighten the plug packing nut. Let me repeat this step, don't over tighten the packing nut!