Paint sanding question

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I'm not sure if I'm in the right sub-forum, but here goes:

I haven't prepped a ride for painting in a few years so I forgot something. How do you keep from cutting through an edge while sanding a fresh coat of paint? By edge I mean something like a character line that runs the length of the vehicle. My 73 Dart has several of those. If I'm not careful it's easy to sand down to bare metal.along that "peak". I think there's a technique but don't remember what it is. Thanks.
ape the top edge of a body line block sand it. Then with that tape still on run tape up next to it. Take the first tape off then block sand up to that edge. Hope this make sense lol