If you wish to sand the surface of your griddle, you can do so, either by hand or using a finishing sander to make things quicker. Select the grit size based on the initial condition of the surface. Gradually increase grit size until it reaches your desired smoothness. You are not trying to remove a lot of metal, so you do not need coarse sandpaper. Start with something like 120 grit and then move up to to finer grits such as 400. You can even go higher if you wish.
Just remember that it is not a smooth surface that makes your griddle non-stick. It is multiple layers of polymerized oil seasoning. The sanding process should be designed to knock down high spots. It should not be necessary to sand out scratches as they will be filled in by the seasoning.
Make sure you thoroughly clean the griddle to remove any sanding debris. Then apply multiple coats of seasoning.
If you Google "sanding flat top griddle", you will find more details and even a few videos.
