Just a note on the muriatic acid. As it might eat away the hard block slowly it will also eat away at the block material at a faster rate. Also if anyone is thinking of using muriatic acid for rust removal , don't do it in your garage, the fumes will kill you before you can get out, also the fumes will rust every piece of bare metal in the garage. Do it out side with a good breeze blowing away from anything that you might like to keep, also wear rubber gloves and eye protection . I other words, muriatic acid is some bad $hit.
That's new to me, I have used MA for years on all types of steel-cast iron, never saw any metal removed, what looked like metal removed was what the rust had allready eaten. Always use outside keep in closed container.
For the OP, put a few drops in there and see what happens, never know unless tried.
