I have a new customer whose lawn has a substantial amount of old crabgrass and very thin on turf grass. The customer want's his lawn re-seeded. I was going to thatch the lawn, put top soil down and re-seed, but when can I treat with a pre-emergent crabgrass control? I know the pre-em can kill the seed. Do I seed in april and put the pre-em down in 8 weeks or the other way around??