When a car with a few scratches comes in to the shop at college i`ve always been told to use 3g rubbing compound and use it in three stages with a air powered or electric buffer.
1 APPLY NEAT COMPOUND AND BUFF STRAIT OFF
2 USE A MIXTURE OF COMPOUND AND WATER (50-50) AND BUFF
3 USE THE SAME WATER AND COMPOUND MIXTURE (80-20) AND BUFF
if the scratches are two deep for this use 1200 grade wet and dry used wet and repeat procedure with the compound.
I hope that this makes sense and is some use to you.