The idea is the clay bar is supposed to be used on a surface BEFORE there is any wax on it. If you have wax on there, then you need to "strip" it all off. Just use dish soap and warm water. You'd be amazed at what that'll take off. BTW, in case anyone reads this that doesn't already know, don't use dish soap on a regular basis unless you plan on polishing/waxing your car every time you wash it.
In the case of the clay bar, you WANT everything removed. A hint for the clay bar, don't do it in the sun, especially if you have a dark colored paint. The cooler it is on the surface you are rubbing, the better. Otherwise the clay will stick to the surface, when what you want is to get the contaminants to stick to the clay.
As others have stated, DON'T DROP IT. One bar is good only for about 2 whole cars, then needs to be replaced. But if you take care of the finish, then you only have to do it once. It's also nice for removing stubborn bug guts.
I've been AMAZED at what that bar system can do. I saw it turn a Firechicken from what looked like an OK paint job, to darn near show quality.
For a Mustang, expect it to take you a solid 3 hours.