I used fine grit sandpaper, 600 grit up to 1000 grit with a flat backing block or file. Then you can polish the rails without rounding.You also need to polish the bolt where it runs on the rails-top,bottom and outside edge but not the back of the lugs. Also the ejector groove and parts of the bolts face where the brass rides up. Another area needing attention is at the bolt handle. As you open or close the bolt, it rides up a ramp on the frame to cock the firing pin. Extractor claw also needs some attention, rounding sharp edges and smoothing it out to slip into the groove in the brass. I think I detailed the areas in a post way back called CZ 550 feed work or something similar. With very fine sandpaper, it is pretty hard to mess anything up and you can target specific areas rather than the general plan of pouring some abrasive paste into your action and working it around but that is just my opinion. I should really take a little time and make a video or maybe just some decent pictures.