On my 84Z28, on the middle of the door is a conture running from front to back, its just a part where the door sticks out more. There are like 2 little rust spots on this part, how can i sand them without getting a flat spot in the conture?
If you are replying to the body line that runs down the middle of the car just sand it as usual. Unless you stay there for an extremely long time in the same spot you will not change the contuor of the sheetmetal
If the rust is not pitted you could use a DA and 80 grit sand from right to left , and move up and down overlaping abought 1\2 inch after the rust is gone sand with 180 and you should be ready to prime