Question about using a magnesium float on a garage floor pour last Tuesday. The finish came out way too open and porous.

It was a 30 yard pour for a two car garage. I switched from my usual aluminum darby to a brand new magnesium float after the bleed water was gone, thinking it would give a tighter finish. The surface ended up looking sandy and rough, almost like it sucked the fines out. The homeowner in Boise was not happy. Did I use it at the wrong time, or is magnesium just wrong for that mix design? What's the trick here?

4 comments

4 Comments

uma6592mo ago

Check the actual air temperature that day. A mag float can pull moisture out too fast if it's hot and dry, leaving that sandy look. You might have been fighting the weather more than the tool.

hayes.wade2mo ago

You switched to a brand new magnesium float on a 30 yard pour. That's your first mistake, they're way too aggressive right out of the wrapper. You basically sanded the surface with it. You have to burn a mag float in on a couple smaller jobs first to knock the sharp edge off the blade. You pulled all the cream right to the top and left the aggregate exposed.

garcia.mila2mo ago

That's a common tip but I've never had an issue on big pours. The key is keeping the float flat and using a really light touch on the first pass. If you angle it even a little, sure, you'll cut in. I always use a new one straight from the supply house for a finish this size.

perry.jesse1d ago

hayes.wade nailed it about the new float being too aggressive. I did the exact same thing on a big pour last year, brand new mag float straight out of the wrapper, and it left that sandy look you're describing. It's like it grabs at the cream and pulls it all to the top, leaving the aggregate exposed underneath. You really have to work those new ones in on a few small sidewalks or a pad first to knock down the sharp edge. I also think you might have been too late with it, like you said after the bleed water was gone. That's when the surface is already setting up and a fresh float just tears it up instead of smoothing it out. I bet between the tool and the timing, that's what did you in.