Hi Jack, would be best if you can remove the machine marks or you will need more primer coats to hide them.
Which way are you sanding ? you should sand with the grain so sand up and down and not along the edges.
Get some 120 grit and finer paper and you should remove them pretty quickly.
Worth grain filling the body before you put the primer on. This will fill all the tiny pores and lower points. The more preparation you do now will pay off in the long run. You will need less primer and colour coats.