If you dont have a handy cement mixer, take it outside onto some hard ground (like a driveway), dump ito on and old sheet, then rub coarse sand into it with the sheet, this should take some of the surface off. If it's mega greasy, which old maille often is, I recommend
gunk which is an industrial de-greaser. brush it on according to the instructions, and let it do its work, then clean it off thoroughly! You may want to leave some grease on there to protect it against the elements.
another option is to hang it up and srubbing it with a coarse brush to get the surface rust off.
Hope this helps!