When it comes to overspary, the clay bar is only good on certain finishes. For general removal and cleaning for this situation, I use 3M Cleaner Wax, it gets rid of most anything.
this is a good restoring wax as it protects and cleans, but once you get the surface smooth again, I recomend finishing up with a good gritless wax or polish. If you like 3M stuff, follow it up with the finishing glaze from 3M.
i personally dont like the mother's clay bar, its kind of soft, I like the one from 3M, Griot's Garage, even the zaino's clay bar is decent.
Also make sure you make a nice FLAT pancake when you use it. The surface area and lube are critical.
I am kind of a detail nerd, my one buddy owns a paint shop, the other is a 3M automotive rep. That and I impulse buy every detail product when it comes out thinking there will be a miracle wax one day.