Pre-emergent crabgrass preventer forms a "film" over the ground that new
plants can't penetrate. So it should not be used with new seedings! You
can wipe out the crabgrass in the late Summer with liquid crabgrass killer.
It works quite will, but leaves your lawn bare! So, be prepared to plant
new seed about a month after the two or three applications of the crabgrass
killer. September-October is ideal for new grass seed.