How do I know when I've sanded down far enough?

[brrymnvette]

Just spent about 20 min doing a little test piece to see if it would be easy to see when I got down far enough. As you can see, the car is Maroon and it's a little hard to tell. I used 80 grit on most of it, and 40 grit on a small section to see if I could see the bare metal easier.



Any tips you have are appreciated.