And of course it all depends on what is there to work with? If the
lawn is full of undesirable grasses and weeds, even if you deal with
the weeds, you're left with crap. IMO, in many cases it's easier to
start over and do it right. Another approach would be to get through
this season using weed/feed, then see what happens. You could re-seed
in late summer/early fall, which is optimal.