What kind of weed killer? And to kill what?

What do the directions say for how to use it?

Where in the world are you?

When do you plan to sow seed?

Depending on where you are most of those knowledgeable about proper grass seeding will tell you to sow seed between mid August and mid September in the northern tier of the USA. That gives the seed time to germinate and grow before the cold winter temperatures begin. Other area may have longer time frames and the southern tier of states have all year to seed.

kimmq is kimmsr

I am in southern,nono

That does not help much. Southern Connecticut? Southern New Mexico? Southern Sudan?

Most all "weeds" will be dying out in the fall so why spray a plant poison on something that is already dying? Plant poisons are meant to be sprayed on actively growing plants.

kimmq is kimmsr