Many of us rodbuilders have Swiss made Grobet files, cut #4, #6 and #8 used for final fitting of ferrules. Coupled with a .0001 micrometer male slides can be fine tuned for a great fit. As beautiful as these file are the valleys of the file fill up with metal filings and render themselves ineffective very quickly. Regular brushes and file cards are too coarse to clean out the valley of the file. Some people advocate loading the file with soap or using chemical to clean the file.
The root of the problem is the number of cuts or teeth in these files, the #4 has 173 tpi and the #8 a whopping 295 teeth per inch. The teeth are so fine it's tough to get anything in the valley. I have a simple way of cleaning the files. Find a square piece of aluminum either 3/8" or 1/2". Rest the file against a stop that is thinner than the height of the file. Holding the stock perpendicular to the file pattern and at an angle of 30 degrees or so push the stock in to the file, quickly the edge of the stock acts as comb and forces the filings out. After a time the stock wears so roatate it to a new edge. When all four edges are worn, trim off 1/8" on the stock and you have four new corners. It's good practice to clean the file before it gets fully loaded. Use a loupe or magnifying glass to see how much crud there is in the file. I hope you give it a try and when it works you can hug a St Bernard or yodell or better yet, treat yourself to some Swiss chocolate.