Wet sand by hand unless,you have lots of body work experience
There is only 0.020-0.030 worth of gel, you want to be easy on sanding. You only need to remove the layer that of oxidation
Get a harbor freight variable speed buffer/polisher
Look at getting seats made at your local upholstery shop.
TACO, MMC, Springfield and others make better hinges and brackets, but not necessary seats
Wise, and others make seats,
The bolt is an M8 x 20mm
That means it is not a crank bolt.
Could be a coupler to flywheel
May be an inspection cover bolt
May be an accessory drive bolt
May be an accessory bracket bolt
Spend $11 on a borescope for your phone and check the coupler
