Chalk or soapstone, I personally prefer soapstone, should always be used as it helps prevent clogging of the teeth and helps clear the chips for the next pass. A lot of applied pressure and clogged teeth will not only gouge the work piece but it will be the death of the file! I have watched people work their rumps off trying to file something while the file was barely cutting and never once did they clear the teeth! Not only does clogging cause severe scratching/gouging of the work piece and damaged file teeth but the clogging holds the file teeth up off the work piece and keeps the teeth from cutting properly. Light pressure with the file held at the proper angle along with frequent clearing of chips will not only make the job go faster but will result in a much cleaner finish, I normally keep a small block of hardwood next to the work and just a light tap of the file on that wood will normally be all it takes to clear the chips from the teeth. When stubborn clogs do happen they need to be removed immediately as continuing to file with this condition will cause damage to the file teeth and make a mess of the work piece.