Kill it all and start over!
Expose bare soil
Kill off patches of lawn with nonselective herbicide in the fall if more than half the area is weeds. When it's safe to replant (check the herbicide label), soak the patch with water and rake off dead grass and thatch to bare the soil.
While we all admire those who relentlessly defend their turf against crabgrass, there comes a time when the best strategy is to give up. That time is when your lawn only has 30 to 40 percent desirable grass left in a given area and the rest is lost to crabgrass and other weeds.
Begin by killing all the vegetation. On a low-wind day, apply a nonselective herbicide that is approved for lawn use, like Round-Up or Kleen-Up. Follow the label directions exactly. Depending on the product, weeds and grass will die and dry up in five to 14 days following application. Then renovation can proceed.
Thoroughly soak the area to give your new grass its best chance for a good start. Check your watering depth by pushing a spade into the ground and pulling it back to get a deep view of the soil. If the soil is moist to a depth of 6 to 8 in., you're ready.
For patchy bare areas and turf-free areas up to about 8 ft. square, use the spade technique for seeding (Photo 2). It's very effective, although it would be slow and tedious on areas that are much larger. Scuff up the dead vegetation with a rake and, using a spade, make 1/4-in.-deep furrows about 2 in. apart. Broadcast your grass seed, then flip a rake upside down and knock the seeds into the furrows. These furrows ensure that the seeds will make good contact with soil; they provide some moisture-retaining shelter as well. Then be sure to keep the seeds and soil moist. Continue to baby your new grass until after its first mowing. Do not apply crabgrass preventer to freshly planted areas.