That's the mantra an old body man taught me:
#1 - Get the Shape
#2 - Get it Straight - (coarser is straighter!)
#3 - Make it Smooth!
He told me the biggest mistake guys make is to make everything smooth before they get the right shape and get it straight...consequently they get really nice smooth waves!
For compound curves, use a flexible (Plastic) longbord also available at your auto paint supplier.
...and use that cross hatch technique as described above!
Shape - Straight - Smooth!