One of the most effective concrete etching products available is HD 24 or Tek Gel for Profiling. The results are similar to shotblasting or grinding the floor at a fraction of the cost. As the name implies HD24 is 24% acid and Tek Gel for Profiling has 20% acid. Comparatively most liquid acids are below 10% before manufacturers suggested levels of dilution.
HD 24 will profile the surface to about a level equal to 80 grit sandpaper and the Profiling Gel to 120 grit.
If you are looking to clean concrete of mold, algae or rust these products have no equal. They are highly effective as you can see in the photo. For epoxy coatings in a garage they are ideal. The profile created is exactly what you need for coatings to stick. If you are planning on using a liquid acid, you can count on coating failure. The purpose of etching is to increase the amount of surface area for epoxies to stick to. Liquid acid solutions are completely ineffective.
These gels are very safe to use. You can get on your hands without injury though it’s always recommended to use gloves. You can use inside as well as out side. For indoor use, be sure that the area is extremely well ventilated.