I did it indoors the first time around and (I believe) I'm still alive and kicking, so I'm not too worried about the fumes, but I will basically be outdoors next time round. Thanks Rick, you definitely can't be too careful when it comes to working with arsenic. I'll leave the file marks alone then, but I will try to scrub off the stubborn blackened portions of the old warangan that refuse to budge with the coconut husk. If the mranggi turns out successfully, I'll be back with pictures.
Alan, definitely good to know the ideal way to go, even if I can't get it 100% right. I might do it up on the rooftop, but in the shade (not in direct sunlight) so there is no glass to worry about. There's a tap up there too, so I'll have a source of water with which to wash the keris off as I'm working.