Change to a liquid.

For St Augustine lawn [assume its floratam - most common variety]

You can get Atrazine at walmart; however you must read the directions on
temperature. The brands that come in the white bottle have a white
powder-like stuff in it which can dry and plug up your sprayer bottle

It can be applied no more than once a month. Lawn only; use a
glyphosate like roundup in gardens

Calm-No wind!!

Its effective only when they are growing.

Its best to apply in the evening, [2nd choice-early am] 2 days before a
rain [or using your sprinkler system]. Figure out your square footage.
I use a hose end sprayer [labeled for weeds]- the simple kind like at
walmart

Other grasses - weedbgone [the big refill tells which grasses are safe]
And it comes in a bottle that you can attach to your hose. Strong
stuff!!

Chances are there are oodles of seed just waiting to germinate.

I have a pair of rubber boots just for weed spraying