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.


I've always puzzled over why John Thorp called for using a pencil to
mark lines in his "building the T-18" articles from the mid 60s. Surely
they know about it then?

John