When is straight ...straight enough?
When it's straight!
It either is or it isn't!
I was taught this mantra by an old bodyman:
"Shape - Straight - Smooth"
Shape: Do your metal repairs then hammer & dolly to the correct shape.
Straight: Cross hatch with a body file and more hammer & dolly to straighten the panel. Shoot your primer, then crosshatch with a longboard to get the panel perfectly straight. You may have to shoot several coats of primer as needed. alternate light/dark as mentiond above.
NOTE: "Coarser is straighter" use 24 grit He said: "The scratches don't lie!"
Smooth: Once the panel is straight, then work your way up to a finer grit until you're smooth enough to paint.
The biggest mistake that amateurs make is they start the final step of smoothing BEFORE the panel is straight resulting in "happy sailors" ie nice smooth waves!
Shape - Straight - Smooth