Rear quarter body help needed

Looks like you have some good ideas, so just a quick way to sand mud when you get to that point
I start when sanding the first few coats with a hand block and 36 grit after I apply the mud I wait till the mud is still a little soft not to the point of it gumming up your sand paper but it will cut the sand time in half and save on sore arms. just a thought.