Is this the "new" P21S product?
I don't know anything about P21S, but I have used S100 Carnauba which many people seem to think is/was the same thing as the "old" P21S.
It is applied to a section and immediately wiped off. You don't let it dry to a haze. I have no idea if it would be difficult to remove if you did let it dry, but there is probably a reason they recommend against it.
The turning black sounds strange. The S100 didn't leave visible residue in cracks, around emblems or on plastic trim. That was one of the things I liked about the product. I think having to tape off your vehicle before waxing it is something that shouldn't need to be done.
I don't remember any residue from the S100 turning black and I'm sure I didn't always get 100% of it removed.
It was a good product for a wax and if I hadn't switched to using sealants, I would very likely still be using it.
It has decent durability, takes very little to do a vehicle, (I would guess a jar would do 30 to 50 applications), and the S100 was about 1/2 the price of the P21S.
Both P21S and S100 make a paint cleaner that is a good prep for their waxes. It isn't a swirl remover and has very little if any abrasive action.
Like any product, reading and following the instructions is a good idea.
