While washing my car I notice big globs of tar on the lower front doors. So with soapy water on the car I used the clay bar and it worked just as well as with the bottle of spray which comes in the kit.
Since it is probably only soap and water anyway, I don't have to buy the stuff anymore, just use the soap while washing.

By the way, even though our mud guards look good, I find they are too small to be of much help.