The Decking fixture is for files only no belt sanders or Dremels. For decking the top of the frame and dust cover use a cheap Home Depot 8″ file or equivalent. The fixture has hardened rods on the edge that will keep you from filing to low. File across the frame keeping the file as flat as possible. Do the same to the lower section as well. Use a straight edge across the fixture to check on your progress. Make sure the top is as flat as possible and check that you have removed the same amount of material evenly across the frame. To remove file marks sand upside down on a hard flat surface with 200 then 400 grit sandpaper and with oil.
This must be done correctly before using the PRC rail cutter.