As the surface has not been sanded, The sanding process will take a long time to finish. You must make the surface flat. Using a power sander will do the work much faster, but you can use your hands as well. If you sand the wood with 120-grit sandpaper at first, there will be slight hills and uneven areas across the surface. So, you will need to sand it further with 80-grit sandpaper, which will be more time-consuming and tiring. Therefore, you better start sanding with 80-grit sandpaper from the beginning even though it takes time. When you are done sanding, you will find that the surface is flat and smooth to the touch. You need to check whether there are still any uneven areas or hills on the surface