Your sanding setup has a direct impact on how difficult this task is.

I use Mirka sander, he’s vac, and mesh paper. 36 / 40 grit. The combination doesn’t load up, is low fatigue, and you save money on abrasives.

With that setup you need to remove the dust from the surface from time to time because leaving it on will cause the part to load up. Also, wear jeans your don’t care about and run the sander over your leg to remove the dust from the paper. If you see pig tails in the surface your loading the paper up and destroying its effectiveness. This is what makes the mesh so powerful, it doesn’t load up.

Is it possible that the black stuff is coal tar residue from an old bottom job?

As another poster mentioned a 7” sander grinder (rotary) with a soft backing pad can be a great tool for quick stick removal if you have the skills to keep the surface even and don’t have access to better equipment like I listed above.

Cheap sand paper is not good, you’ll end up spending more than you would on a high grade industrial product over the job and have more time / wear and tear on the body.

In old office chair with adjustable height arm rests is a life saver for this type of work. I also have a milk crate for lower sections. The chairs nice because you can lean back instead of squatting down working overhead in a tuff position and often times get the arm rests in a position where your can support your arms while sanding.

I do believe the osha standard is something like 15 minutes off for ever 15 sanding as a vague rule but the Mirka has Bluetooth connect that measures vibration exposure times and you can sand longer.

I never paid attention to it and just pushed through it and now have severe issues with tendonitis and nerve issues in my arms and hands. It was not worth it. As a hobbiest I don’t know it will hit you that hard but doing boat / bodyshop work for years as a sideline has taken its toll. I wish I could go back and buy the Mirka 15 years ago so I still had feeling in my fingers and no pain in my arms.

I would go back and do a lot of things different I guess. It was also mind over matter and pushing through pain. Well now I can’t push through the pain and need help with a lot of task I used to do without worry.