I never worry about the white border as I make matts for all my mural prints.
Just one more finikiy thing to worry about getting right when printing.
Did I mention painting the board black, this will help.
also all the space around the enlarger should be black and you should mask out the negative.
These steps will reduce flare.. the cut out mat-board is not a good idea as you will find flare on the edges from the white board, if you insist on the mat board mask go get Black board that when cut the core is black as well so as not to give you added flare.

Quote Originally Posted by Chris Lange View Post
I cut a mat-board mask for the borders, too, so the paper will not have any bleed-over of exposed silver outside of the image area. Thanks again, Bob, you're a life saver!