How do you properly and safely use muriatic acid to take a sealer off a concrete garage floor so you can apply Rustoleum paint on the floor?

First, depending on exactly what sealer was used, it may not even take it off successfully. Perhaps a test is in order for you. Using muratic acid is not rocket science. Every mason uses it nearly daily, and it's more pure form is used in your pool. The key things to remember are safety glasses, proper ventilation, and "add acid to water". Read your acid documentation. Your application probably calls for 10% acid, which for me meant adding my gallon of acid to two gallons of water. I used a 2 gallon water can with a flower water sprinkler head on it. Pour it on small areas, say 3' by 3', and scrub the acid area with a plastic brush on a LONG handle. Do not bend over near the acid with a hand held brush, as there is no way to avoid breathing the vapors. For my floor, after a couple of minutes you could visually see that the acid was spent, as when you brushed it, no futher bubbling occurred. Flush well with water. The floor should feel like medium grit sandpaper if properly done. Some areas of my garage had some type of sealant. It was easy to tell when I had not gotten through it, as despite the acid wash, the concrete was still smooth. I reapplied the acid a couple of times and it worked eventually. I would recomend a mask with an organic vapor filter, but didn't use one personally. Either way get a couple of good fans for ventillation. More input from others: * I don't like the stuff, since I ahd a very unpleasant experience when using it. I know you have to be extremely careful how you dilute it, and what sort of container you use, etc. It has a lot of contaminates in it so you need to have adequate ventilation. Probably your best bet would be to call the manufacturers, if they have an info 800# most do. Or you can check out bobvilla.com which has some really great info. on lots of "house stuff." * Muriatic Acid is also used to remove rust from steel ... and does a great job. But after the rust is gone it will continue to dissolve the basis steel, so be alert and when the rust is gone ... remove the article. Also, fumes from the acid, though you may not see at once.. will cause eventual surface rust to any steel parts in the close vacinity.