I find it fascinating that the repair of cast iron provokes such
a divergence of opinion across a group of uniformly fine craftsmen.
I have found this to be the case not only in piano work, but with
the people who restore old machinery as well.

It's always been controversial, apparently.  I've got an ancient
welding book somewhere that shows the repair of engine block from
a 1928 Marmon or something.  They show how they built it up here,
and here, and banged on the welds with each pass, and how it's now
in great shape.  So, yes, it's practical.

Then they emphasize that this was done somewhat as a stunt by the
Lincoln arc welding people, and that the repaired block can still be
seen at the Lincoln Arc Welding Foundation (there is such a thing)
in Cleveland, Ohio.  So, no, it's not really practical.

Still later, they list a selection of cast-iron and nickel electrode
rods that are developed 'specifically for the repair of machine
castings.'  So, yes, it's practical.  And this is all in the same book.

My old man worked for an outfit that published a welding magazine.
Those guys contended that if it's made of metal, it can be welded,
and there is some evidence that they were more or less correct: they'd
discuss things like techniques for welding the wings onto a fighter
aircraft with an electron beam.

My own experience has been that I can't do it by welding, brazing,
or prayer but that there's a tractor-repair outfit near Starkville,
Mississippi, whose owner can weld any exhaust manifold or cylinder head
into newness.

There was also something called 'bronze welding' which was supposed to
be great with cast iron.  Don't recall just what it was supposed to be,
but they emphasized that it wasn't brazing.

Mark Kinsler (gonna use 5-minute epoxy!)
Lancaster, Ohio, USA
http://www.mkinsler.com/