Finishing knife handles.
Knife handles are a difficult thing to finish. They will be handled, so soft finishes are not viable, and even the highest mirror sheens will dull with use. Knife handles are also often made of extremely dense, oily, figured or stabilized woods. These types of wood offer different finishing issues to other applications.
From shaped to finished, Sanding is the most important part of finishing.
One major factor to remember is to keep the wood cool. Just as steel can overheat and ruin its temper, wood that gets too hot will scorch, crack, check, twist or change in color. For handle work, i highly recommend buying the very cheapest AO belts and paper and changing out relatively often, always using fresh, sharp grit to lower the risk of these issues. Its also important to wait a little while sanding to allow the mass of the wood to cool down and for the heat to travel within the piece. Uneven heat causes cracks and checks. This can also be improved by using a machine sander at ~25-35% speed. A variable speed sander makes getting a good finish much easier than going by hand or at a high speed.
A good finish starts with good sanding. After the handle has been shaped, you must remove each layer of scratches carefully and completely before progressing up the grits. Left over scratches will be almost impossible to remove at higher grits.
Assuming handle shaping ended 60-80 grit, The first step should be to go over the handle with an 80-100 grit belt to make sure there aren't any deep scratches from the initial shaping stage. This is also a good time to apply thin CA glue to any small cracks, checks, pin holes, voids, soft spalted areas and anything else that is vaguely worrying. My favorite brand is https://trugrit.com/product/insta-cure/, its a water thin CA glue and will wick into any size gaps. I strongly recommend buying the debonder as well, https://trugrit.com/product/un-cure-debonder/ If you have not worked with water thin CA glue before, it is amazing how fast you can glue your fingers together. Run a thin bead over any areas of concern and allow to cure. Depending on the depth of the crack, you may see bubbles as the superglue is wicked into the voids. I recommend laying down multiple thin layers. By filling the crack once or twice with CA, and if it still wicks glue down allow 10-15 min to cure before adding more layers. While this does add time, it prevents cracks from spreading, and will yield a much more attractive and consistent final product.
Once the handle is at an even 100 grit, progress up the grits as normally, common grit steps are grits of roughly 120-150 to 180-220. At the 220 stage you are really finishing the handle. Stray grits or pieces of sawdust can scratch the wood or damage the finish. I recommend rinsing the wood by hand in warm water, just to remove any dust or bits of leftover grit before finer grits are used. Strong compressed air or a tack cloth can both also be used, but i find a quick rinse is the most effective.
Continuing to sand to roughly 320-400 finish. At this point, i recommend stepping down a grit to ~300 and sanding the handle by hand. Examining the piece all over, you may find scratches that had been missed. It will be incredibly difficult to remove these scratches moving forward, and if they can not be easily removed with the 300 paper you may have to re sand the piece at 220 grit and work your way back up.
From here, you can continue jumping up the grits until you reach your desired finish.
Depending on the fineness of the grain of the wood, different woods can be sanded to different levels.
Large pored woods like walnut, oak, or wenge can be sanded to ~400-600 grit before no real improvement is made.
Medium pore woods like stabilized Maple, koa, acacia and afzelia can be sanded to ~800-1500 grit.
Very fine pore woods like ebony, rosewood, or desert Ironwood can be sanded to 2200-5000 grit.
With a well sanded handle, the next step will often be buffing. Buffing allows a much glossier finish on wood and can make a huge difference. Many of the same rules from sanding apply to the buffing process.