The bond
Why does concrete have to be ground before it is coated?
A coating grabs a roughness in the top of the slab rather than a smooth surface, and grinding is what puts that roughness there.
The short version
- A coating bonds to a profile, which is a roughness in the top of the slab. It does not bond to a smooth surface.
- Diamond grinding is what puts a profile on concrete.
- A hard-troweled slab is the hardest surface to bond to, and most newer garages and basements have one.
- Grinding also takes off oil, old sealer, paint and adhesive that a coating would have had to stick through.
- The machine runs with a vacuum on it, and the floor is vacuumed again before anything goes down.
What a coating sticks to
A coating does not sit on concrete the way tape sits on a table. It keys into a roughness, a texture with somewhere for the resin to run down into and grab. That texture is called a profile, and a slab poured and finished for a garage does not have one. It was made smooth on purpose.
Grinding puts one there. Diamond heads take the top off the slab and leave a surface that feels like fine sandpaper, open enough for the coating to sink into and lock. What you end up with is not a coating lying on the floor, it is a coating hooked into the top of the concrete.
Why a troweled slab is the hard case
Most newer garages and basements have a hard-troweled floor. A power trowel run over fresh concrete presses the surface tight and closes it up, which is what makes it easy to sweep and easy to keep clean. It also makes it the hardest thing a coating can be asked to bond to, because nothing is left open at the top for it to hold onto.
That is why the machine matters more than the bucket does. A slab closed up like that needs the top of it taken off mechanically, and there is no way around that step on a floor that has to take hot tires and salt water through a run of winters.
What comes off, and where it goes
Grinding takes off more than roughness. Whatever soaked into the slab over the years goes with it: oil, tire marks, an old sealer, paint, the glue left behind by tile, the film a previous owner rolled on. All of it is material a coating would have had to stick through instead of stick to.
The dust is the other half. Concrete ground dry throws a very fine dust that gets into everything in the house and settles right back into the profile that was just opened. So the machine runs with a vacuum on it and the slab gets vacuumed again afterward, and the dust leaves in a barrel instead of staying on the floor.