TIL the hard way about magnesium vs aluminum floats

Used an old aluminum float on a garage slab last week and it just dragged the cream, left a rough finish. Switched to a magnesium float for the next section and it cut like butter, gave a way tighter surface. Anyone else have a go-to tool that changed their finish game?

3 comments

3 Comments

marywilson23h ago

Yeah, that's a classic rookie mistake. Read an article years back that explained the science. Aluminum floats cool the surface cream too fast because they pull heat out. Makes it set up before you can work it right. Magnesium stays closer to the concrete's temp, so it glides and doesn't stick. Makes all the difference on that initial finish before you even think about a trowel.

dakota41522h ago

Wait, aluminum floats pull heat out? I never knew they actually cooled the surface down. I always just thought magnesium was lighter or something. That explains why my first few slabs with an aluminum float came out so rough, like it was dragging through sand. I just blamed my timing. So it's not just about the tool feeling different, it's literally changing how the concrete sets. Mind blown.

sage_green20h ago

Actually, the heat transfer thing is a bit of a myth. The real difference is the friction. Aluminum sticks to the wet cement paste because it's a softer metal. Magnesium is harder and has a lower coefficient of friction, so it glides over the surface without grabbing. That's why it works better for that first finish pass.