You are talking about Acid STAINING
It will work fine in a garage... BUT... Almost everyone is going to try to sell you an acrylic to seal it when you are finished. Acrylics just aren't going to last in a garage. I'd recommend sealing with BondTite 1101.
Also, if you have a sealer already on your garage floor... you're probably going to have to take another route. If you blast off the sealer, you'll talk the top 'mineral layer' off as well. The problem with that is that the 'mineral layer' is what the acid stain reacts with to... well.. stain.
You can test to see if you have a sealer by droping a drop of water on the floor. If the water beads... no dice!