The best way I have found is to use an angle grinder, but that is very loud and will attract a lot of attention. On the flip side it is a lot faster than a Rub Brick so you could maybe knock it out in 10-20 minutes opposed to an hour or more. It depends how chunked out it is, if its not terrible taking the chunks off, and then using a quick dry clear coat works the best. Put a few coats for the best results, I've done this to several brand new ledge spots/curbs and if done proper they grind better than angle iron.