In article ,
"Morgans" wrote:

"Jim Logajan" wrote

Why can't you just clean off the pencil marks?

Also, doesn't a galvanic reaction require an electrolyte between the
metals, so if you didn't erase or clean off the pencil mark but primed
and/or painted over the aluminum surface, no salt water or other
electrolyte could get in there to produce the galvanic reaction?

The danger of using pencil for aluminum is not a "maybe" kind of problem.
The pencil works it's way into the molecules, and can not be cleaned off,
completely. It is an accepted fact, known to materials engineers as a
unacceptable practice. I don't know if it is really a galvanic reaction, or
something else, but people *way* smarter than you and me have proven the
problem. The metal will become brittle at the pencil line, and with enough
stress, *will* cause a crack to start.

Why risk it? Why argue? Use something else to mark your aluminum. Period.

And --- DO NOT USE A SCRIBE!!! Scribes, no matter how careful you are,
will damage the aluminum and create stress raisers. They can also damage
the Alclad coating and provide a path for corrosion.

It is not a matter of IF a piece will fail at a scribe line, but WHEN --
due to fatigue.

The admonition against pencils in aluminum shops also holds for engine
shops. The easiest way to fail a hot steel part is to mark it with a
lead pencil. The steel glows red hot and absorbs the carbon from the
mark, creating an instant stress concentration. An old friend related
how he saw a Lockheed Constellation exhaust manifold with a crack in the
shape of "OK", due to lead pencil.