Yes, a whetstone can be used to sharpen a katana. In fact, whetstones have been traditionally used for sharpening and maintaining the edges of Japanese swords, including katanas.

To sharpen a katana with a whetstone, you will need the appropriate grit size of whetstone and some basic knowledge of sharpening techniques. Here are the general steps involved:

1. Prepare the whetstone: Soak the whetstone in water for the recommended time to ensure it is properly saturated. Different whetstones may have specific soaking requirements, so follow the instructions that come with the stone.
2. Position the sword: Secure the sword in a suitable manner, ensuring it is stable and won't move during the sharpening process. You can use a sword stand or a clamp to hold the blade securely.
3. Select the angle: Determine the sharpening angle you want to use. The traditional angle for sharpening a katana is around 15-20 degrees, but this can vary depending on personal preference and the condition of the blade.
4. Begin sharpening: With the whetstone placed on a stable surface, hold the sword with both hands and carefully run the blade along the whetstone, maintaining the chosen angle. Apply even pressure and make smooth, controlled strokes along the length of the blade.
5. Flip and repeat: After sharpening one side of the blade, flip the sword and repeat the process on the other side. Aim to maintain an even bevel and remove any nicks or dullness along the edge.
6. Test the sharpness: Periodically test the sharpness of the blade during the sharpening process. You can do this by carefully running your thumb along the edge at a safe angle to feel for any burrs or imperfections.
7. Progress to finer grits: Once you have achieved the desired sharpness with a coarse grit whetstone, you can progress to finer grits to refine the edge further and enhance the polish. This helps achieve a keen cutting edge and a pleasing aesthetic.

Remember, sharpening a katana properly requires skill and practice. If you are unsure about sharpening it yourself, it is recommended to seek guidance from an experienced individual or a professional swordsmith who can provide expert advice and assistance.