It is hard to diagnose anything without being able to examine and see what is going on and where. What is the history of the vehicle? How many times has it been painted? Unfortunately, the hood is what we as a driver see the most and because of the large surface in front of us, we tend to focus of it.
Before cutting loose the surface metal front the support structure, I would take/strip off most, if not all the paint, and find the root cause(s) of your distortion. Also, do not use a die grinder or a heavy type of grinder with aggressive grit sandpaper, such as 36 grit. It is too easy to cut into the metal...before you know it you will be thinning the metal. Be very patient, go slow and keep moving your dual action sander across the surface. If your car has been painted several times, I would start with 180 or 220grit until you begin to see a different layer of paint across the hood...definitely stop before you hit the primer. Be mindful of how much your sandpaper is cutting and replace the disc when you notice it isn't cutting easily. Biggest mistake is to apply more pressure; also, you do NOT want to create any heat! As for sanding your hood, or any large flat sheet metal, just guide the sander...place your fingers around the base and not on top. At some point in your sanding, you will begin to see evidence/patterns of the high and low spots. When this happens, you will have to decide whether to continue removing more paint/layers.
The next step is to address and resolve the wavy surface with high build primer and proper sanding. Application of two layers of high build primer is what I would recommend...let dry thoroughly. Then, apply some guide coat and begin sanding with 320grit and progressively switch to 400 then 600 grit. Again, as you sand, just guide the sander and use cross hatching patterns. Once the guide coat is gone, move to the next area. I like to work in 3x3 foot sections.
Hopefully, you will not need to mess with disconnecting the hood from the support structure. The engineers designed this feature to provide a sound foundation for the large sheet metal.
As one last bit of encouragement...be patient and once you finish, you will be very proud of the work you performed to get your ride to this beautiful condition!
