hmmm-i was going to suggest planting and tilling one or two cover crops but sounds too rocky. The other thing that has worked for me is to water and fertilize the weeds, then spray with round-up-i find i can use it half strength if i add a little spreader/sticker. Do this two or three times and then plant your grass. Round-up only works if you get it on the leaves and is supposed to break down in soil so you should be able to plant grass no problem. If you plant sprigs or plugs you can use a preemergent until the lawn is established but pretty $$ and won't work with grass seeds. Good Luck.

what's a spreader/sticker? Sorry I am totally clueless with gardening! Thanks for the info.

it helps whatever you're spraying stick to the plant. You could use dishsoap but it gets a little foamy. Ask for it at your local ag. supply or maybe home depot. Only need a drop or two.

A wonderful weed-killer that you can get at some farm supply stores is pure clove oil. It kills 'em dead and fast, smells absolutely wonderful, is non-toxic, and will allow grass to grow later once it has broken down after a couple of weeks.