just want to echo some of the comments here from some experimenting this weekend.
my conclusion: these type of products work well, but on a heavily contaminated car, you still need to follow with clay.
Picked up a scrap hood from the body shop dumpster (don't judge me) just to play around with. Got it home and found it was loaded with above the surface contaminants.
I did half with the nanoskin using my 'normal' technique and amount of time spent. significant improvement in the surface, but did not pass the baggie test. went over it with clay magic blue which pulled up a fair amount of crap still
I did the other half with the nanoskin, but spent a lot more time, and even a little pressure which I normally wouldn't do. once again, big improvement, but clay magic blue was still needed (pulled up a little less this time around but still was significant enough that it had to be done)