Sand Off, Then Re-Draw Pencil Lines?

In a comment, CEM asked:
"Chris, I do not understand why you sand the line off of the tube and then state: "Re-draw the lines back on the body tube." Why re-draw them onto the tube after you sanded them off?"
Thank you.


Good point, CEM!
The pencil lines are first drawn for a sanding reference, for slightly roughing up the tube for fin root edge gluing. 

In the picture I am using a sanding block. A couple passes with the flat block (passes made at the same angle) makes an even roughed area in a straight line. 

As the pencil lines are "erased with the block I know I'm sanding an even width strip. 

With the lines gone they are re-drawn on the newly roughed up area.