Originally Posted by Valk
Not saying you don't already know this, but, Sept is a WAY better time to plant seed as there will be ample time for the roots to take. Even though our Spring is WEIRD this year, generally Summer's heat comes on a bit too rapidly for Spring seeding to 'take' -> like it does in Sept. Spring seeding is best put down in Feb...though it will not germinate until the soil is warm enough.
I would even suggest watering the area throughout the summer and repeating the RoundUp. An area bad enough to require 2,4-D and RoundUp is pretty weedy. My preference is to kill weeds in an unplanted area rather than in a new lawn.