Correct me if i'm wrong, but mowing and spraying with Roundup sounds like starting over fresh. Overseeding to me is adding seed to a plot that is partly established and needs bare spots covered, or adding something like annual Rye to help deter grasses and give cover to what is already there. I overseeded 4 plots (small) that was getting some grasses with Rye. Another plot that the grasses had taken over I retilled three times, about a week apart and reseeded with clover and annual rye.

Russ