[stevehertz]

Like the OP I have been using this method in my guitar setups services that I provide for many years. However, instead of using baking powder, I use bone filings. Firstly, the best guitar nuts are made from bone so using the same material to correct a badly cut nut slot is apt, and secondly it is a very good material for the purpose. Whenever I do any filing work on a bone nut or saddle, there is always a small heap of bone filings that are carefully collected up and kept for this purpose. You can of course get a large beef knuckle joint bone and take a hacksaw or a file to to it to get a good supply. In terms of using the stuff, in a guitar nut I fill the slot with the ground bone then tamp it down with a screwdriver blade to increase its density before dropping on a few drops of thin superglue using a piece of wire. Obviously it will be more difficult to tamp down filing in a more 'open' scenario, but where you can, all the better.