[elitecustombody]

OP, use good masking tape to mark the lines and block up to it,when done blocking, lay another strip of masking tape right up against the old tape and block the other side,when done blocking,you shoud end up with very sharp and crisp line,if it's too crisp,roll over it few time to your liking

use straight edge wherever you can to make sure you have no dips in the panel.That can happen if you use short blocks, so try using long blocks where possible, I recommend 2x1/4x24 aluminum stock wrapped in masking tape few times,or stop by Home Depot and pick a paint mixing paddle. I never trust foam blocks,even if they cost $50 a piece ,when blocking, do not push,let the sandpaper do the work and after few strokes,change direction