Classic case of Ready! Fire! Aim! I realize I'm going to sound a little cranky with you, but hopefully you and others will never make the mistakes you already made. SPRING IS NOT THE TIME TO RENOVATE A LAWN. Was I shouting? Oh my! Why would that be? Because spring is when all the weed seeds are looking for enough water to germinate. If you put down grass seed, you will have to water the grass seed about 5x more than the weed seeds need to germinate. That means the weeds will grow in first. Since they are weeds, they also grow fastest (the nature of weeds). In your case you are guaranteed to have a full weed lawn by June no matter what you do now. Spraying with Round Up was the main thing you did wrong. Before that you had half a shot at restoring the grass or at least having a sort of okay looking turf. Now? Not so much. But you have to do something. Use the slit seeder and use it with the cheapest seed you can find. Why the cheapest? Because it is going to die anyway in the summer heat. Don't waste your money now knowing that in the fall you're going to do it all over again. From now until fall, take this time to learn how to maintain the lawn. I don't care if it is 100% unwanted weeds, the care is the same to make it presentable. Here is the 1-2-3 of lawn care. Water deeply and infrequently. Deeply means at least an hour in every zone, all at once. Infrequently means monthly during the cool months and no more than weekly during the hottest part of summer. If your grass looks dry before the month/week is up, water longer next time. Deep watering grows deep, drought resistant roots. Infrequent watering allows the top layer of soil to dry completely which kills off many shallow rooted weeds. Mulch mow at the highest setting on your mower. Most grasses are the most dense when mowed tall. Bermuda, centipede, and bent grasses are the most dense when mowed at the lowest setting on your mower. Dense grass shades out weeds and uses less water when tall. Dense grass feeds the deep roots you're developing in 1 above. Fertilize regularly. I fertilize 4 times per year using organic fertilizer. Which fertilizer you use is much less important than numbers 1 and 2 above. I did not create these three steps, although I have adopted them. These are the consolidated wisdom of the ages on this forum. If you follow those three steps your grass and/or weeds will look better than any other set of practices. Then at the end of summer you can spend good money and time to prepare your lawn for 2012. Please, before you do that, write back here to find out what to do. Or spend some time looking deeper into the archives of this and other lawn forums to learn what not to do. There are a few very popular mistakes people make. Your Round Up mistake in the spring is not that popular, but it might be a sign that there is a commercial on TV indicating it is a good idea to do that. Just trying to catch...

Power raking with a fixed blade machine (sometimes rented as a slit-seeder, verticutter, verti-slicer or power rake) can be very beneficial in the seeding process. Take heed of dchall's advice. David. Are you aware that Scotts has come out with a starter fertilizer with mesotrione? (they also have one with siduron) I can't recommend it yet, as I've only just started a small area to experiment.

Using a fixed blade verticutter/slicer, like plug aeration and power raking is a very agressive proceedure and will result in substantial damage to existing turf. (imagine using a carpenter's circular saw to cut grooves every 2", or less depending on the reel, apart the length of your lawn) You need to carefully consider if what you expect to accomplish outweighs the damage that will result during the process.

Generally, it is better to run the slit seeder without the seeding component and bag up excessive thatch. Once already seeded, I think that is hard to do without baggin up a lot of seed in the process that with time will fall to the ground. A long as the dead material is not inches thick, the dead stuff can serve as good mulch. If you have a pile of it in one area, try spreading it around more evenly.