What kind of liquid weed killer, Tryharder?
Most liquid weed killers work through the leaves of the weeds, so you want the weeds to have a good crop of fresh, young foliage when you apply. I would either scarify, then wait a couple of weeks to apply the weed killer, or apply the weed killer, and wait until the weeds are dead to scarify.
If you are thinking of a liquid weed preventer (pre-emergent) apply immediately after scarifying, assuming that you are not planning to re-seed.

Thanks for information Tugbrethil,
I am using Verdone liquid lawn weed killer, I find that I can apply that better, I tend to overdo it with the granule type weed killers, when I have used before.
I am going to use it now then and wait till weeds are dead and then scarify,
Thanks for replying

I've always had better results with the liquids, myself. Best of luck with the weeds! :)