Andrew Taylor asked about a filler for pitted metal.  The technique to
use is the same as in auto body repair.  I also used this method to do
the plate on my Weber Duo-Art grand, so I know it works on cast iron
too.

Clean and strip everything off of the metal, down to bare, shiny metal.
Use rough sand paper, wire brushes, whatever.  The more abrasive the
better.  The more roughed up the surface is, the better the primer
sticks.  If it's rusty, treat the metal with a metal conditioner
(Sorry, I am not a chemist and don't remember what it is, but it
neutralizes the remaining rust).  Prime with a primer surfacer.

Major pits are then filled with "glazing putty", which is nothing more
than a concentrated lacquer-based primer.  It comes in a tube, like
toothpaste, and you smear it over the pitted areas with a plastic
applicator (like a spatula).  When dry, you sand the whole thing down
with 80 grit sandpaper.  Use a sanding block to ensure the sanded
surface is flat.  Prime again.  Use glazing putty again, if necessary.
Sand again.  Continue repeating this until all pits are gone.  The idea
is not to build up a thick layer of primer, rather it is to fill the
pits.

When all the pits are filled, prime one last time and sand with 320
or 400 grit paper.  To ensure that all pits and sand scratches (from
the 80 grit paper) are gone, you can apply one last "mist" coat of
a different color primer, called a guide coat.  This should be a very
light coat.  Sand all of this last coat off using 400 grit paper.  Any
pits or sand scratches will show up because they will remain the guide
coat color.  When done, apply your finish coat(s) and you're done.

I did this to my Weber, and my final coat was Imron (a urethane-based
enamel, with hardener, used for airplanes, trucks, etc.).  It flows
to a wonderful gloss.  I picked a metallic gold color from the color
chips.  I know the purists will take umbrage, but the gold color I
picked is much shinier and brighter than I see on new Steinways and
Boesendorfers, but I think it's prettier.  In any event, with the
proper application of time and patience, this technique will give you
a flawless, pit free finish.

All the products discussed above are available from your local friendly
automotive paint store.  The thick black filler Andrew mentioned could
be glazing putty.  I dunno.

Regards

Ray Fairfield