To start with you should determine if the chisel has to be sharpened or repaired.
Sharpening a Chisel
Clamp the chisel into a fixed table vise so that the taper or bevel is facing up. To determine what grit to start the sharpening process with check to see how sharp the edge is. If it is very dull but still has some bite start with 400 grit wet/dry paper or stone. If you use paper put it on a block to make the paper sit even on the bevel. Make sure to lay the block or stone flat on the bevel. It doesn’t matter if this is a round, flat or “V” chisel. Press down evenly on the block with a slight more pressure to the front or cutting edge side. Use the back and forth motion or round and round motion. But do not rock the block or stone back and forth. A good way to see where you are sharpening is taking place on the bevel is to see where your scratches from the paper or stone are taking place. When you can see that your edge is getting sharper you can advance on to the 600 grit paper and after you have done a few strokes and advanced the sharpened edge go onto the 800 grit. Do the same with the 1,000 and the 2,000 grit papers. After this you will want to use the 10,000 grit paste on the leather strop. You can use a flat strop on the bevel and on the inside use a strop that is wrapped on a dowel. The inside of your chisel will determine the radius of the dowel. Do the majority of the stropping with the flat strop on the bevel side and clean the burr off with the round strop on the opposite side.
Again when you are sharpening the chisel do not rock the sharpener back and forth because what happens is you will set up a convex bevel and it will have very little if any bite going into the wood, To determine if the bevel has a convex area put the bevel down on a flat area if you can see a crown or a convex surface it is time for a repair.
Repair A Convex Bevel
If your chisel is not too hard you can use a good or new file. If the file slide off of it you will have to resort to the slow speed belt sander. Use a course, 80 grit belt, Determine the angle you want when you are finished and work toward that angle. Press the convex area lightly on the belt for a two seconds and determine how hot the metal is getting. To do this you will have to touch the sharpened end to determine the heat. You don’t want it over a hundred degrees Fahrenheit. Dunk it in the water to keep it cool. Also yopu can press on for 2 second and off the belt for two seconds then back on for 2 seconds. Keep on dunking it every onces in a while. Don’t get in a big hurry with this process because you can make lots of work for yourself if the chisel turns brown or blue in color from sharpening. If this happens the structure of the steel has been changed to get it back to a normal harness for this steel it will have to be re-tempered. So go slow. In this case slow is fast. After you have taken the convex out and you have the angle on the bevel you want then you can start with the 400 grit and use the sharpening instructions above. Remember avoid the rocking motion with the sharpener be it the paper or the stone . To do this watch where your scratches are taking place.
Repair a Hollow Bevel Chisel ( Recess bevel on the round chisel )
This will happen if the back of the bevel has been sharpened too much. To recognize this you will see the inside or bottom side of the chisel goes back and the shape looks elongated.
Take a new or good file and file the cutting edge straight. This means knocking the corners back. Then get the bevel you want either with the file or belt sand as above. If you are using the belt sander do it slowly and lightly so as not to cause too much heat in the metal. After the corners have been knocked back then get the bevel you want. Watch the bevel it maybe convex at this point too and this will have to be straightened out also. When this has all been straightened out then use the sharpening method to touch up the edge to where you want it.