Mix the acid - 1 part acid to about 5 parts water. You can strengthen it if you need to, but try the 5:1 first. (5 parts water to 1 part acid). Note, you should always start with the 5:1 or even weaker of a mix first, before going any stronger.
The acid will yellow the concrete and eat away the surface if allowed to sit there for any amount of time at all. So it is extremely important to have a water hose running and ready, or to have a bucket of clean water with a mop sitting in it, on the ready.
You dip the brush into the bucket of acid and water, then you shake off the excess into the bucket. Then you sit the brush down on the concrete and scrub it well yet quickly, being careful to not splash or splatter any acid/water mix onto yourself or your surroundings. Then you immediately remove the brush and place into the bucket or a predetermined safe place area, and then either quickly start rinsing the acid mix off the concrete with the water hose already running on full, or start mopping it up with the mop and the bucket of fresh water.
It is very important to get all the acid mix rinsed all the way off of the concrete or all the way mopped up. You can repeat as needed until you get it scrubbed all the way down.

How To Repair Cracks In Stained Concrete Floors Home Depot

How To Repair Cracks In Stained Concrete Floors Flooring

I would use a polymer modified crack repair material to fix the cracks if you want to stain the concrete floor. This material will accept an acid stain unlike an epoxy crack repair product. Keep in mind the patching material will stain differently than the concrete floor will. For an old concrete floor with serious cracks, gouges, stains and other imperfections, patching and painting alone may not be enough to fix it up. If you're dealing with more significant damage, a.