I worked for a long and agonizing summer as a welder. During a slow period when our steel was delayed (one type came from Belgium), one of the older guys taught me how to braze cast iron. There were a couple of things you could never forget to do and using an oxy-acetylene torch was the best option. Wasn't really all that difficult, but your repair was very obvious...no way to hide it. Slow work if you had a long crack. There were some DC brazing rods where we worked, but he said they were basically junk. This was 1969.
