A second pass, not the first one
A bull float is the first tool to touch the slab after it is screeded, and it is a big one, aluminum or magnesium on a long pole, worked from outside the forms to knock down the ridges the screed left and bring a little paste to the top. A mag float comes after that, once the concrete has firmed up enough to take a closer pass. It is smaller, worked by hand or on a shorter pole, and it goes over the same surface again to smooth what the bull float left and press any small voids closed before edging and jointing happen.
Why the surface stays open
A magnesium face does that closing work without sealing the surface shut the way a steel trowel does. Magnesium is soft enough to glide over the aggregate instead of burnishing it flat, so the pores that let bleed water and air escape stay open. That matters because a slab finished too hard, too early, traps water under a skin that has already set, and the surface that forms over that trapped water is the one that scales and flakes the first hard winter. A mag float smooths the surface without shutting that door.
Where it fits on our own pours
On the outdoor flatwork we pour, a driveway, a patio, a walkway, the mag float is the last hand tool before the finish goes on. We follow it with a broom drag for grip, or with the color and mats if the job is stamped. A hard steel trowel finish is a different job entirely, meant for an indoor floor that never sees ice, and we save that tool for garage and shop slabs rather than anything left outside.