That DA should be fine for taking panels down to shiny metal BUT there are many variables that will affect how well it works. What size is your compressor? Brand of paper your using? How many coats of paint on the car? Is it enamel or Imron? If the paint is thick then 40 would be OK to get it close. If original paint or maybe one repaint a quality paper like Duragold in 80 grit will do the job, you will just need to change it often and be patient.
