I'm with John - old files past their use-by date are ripe candidates for scrapers. Good hard steel. Pick up cheap from car boot sale? Take the teeth off with a grinding wheel, shape and ideally hone the cutting tip to give a sharp edge. Quick look at images on Internet show square ends (like a chisel) but I've found that rounding the end helps avoid the corners digging in. You can always take the file tip to red heat and belt it with a big hammer to reshape; quench in oil (or water if you have to - I'm not a perfectionist...) and again clean up with grinder and then hone to a good edge.