About the cheapest way is to get a piece of glass, tape it to your table saw
or other flat surface, apply coarse valve grinding grit (available in any
auto parts store) and have at it. Virtually all glass made now is float
glass, and is very flat. I know it is none of my business but why not get a
new India combination stone; you will find that it cuts better than the
carburundum. Additionally, you will have one beater stone for knives and
gouges and a flat stone for chisels and plane irons.
"AAvK" wrote in message
news:xTj7d.4011$mS1.1317@fed1read05...

I have an old Carborundum SC dual stone I'd like to flatten, won't spend
that much for a ceramic plate, I know the stone will ruin any file too.

Alex