A faucet valve reseating tool will grind down and resurface the valve seat if it can not be replaced or is actually built into the hardware. Using this tool is similar to the results and idea of using a sander.
Step 1 - Grind
Insert the reseating tool until the grinder is resting on the seat. Turn the handle clockwise and begin grinding down the valve seat.
Step 2 - Smooth
Feel the valve seat to see if it is smooth to the touch. If it feels smooth, you may move on to step 3. If not, keep grinding until the valve seat feel smooth. It is very important that you do not over grind the valve seat. If this happens, you will have to replace the part.
Step 3 - Clean and Reassemble
Once you have made sure the valve seat is smooth, take a damp cloth and wipe away all metal dust. When the valve seat is clean of all grit, you may then reassemble your faucet.
Please note that reseating tools come in several sizes, so make sure you are using the proper size for your job.