The clay doesn't "rub on". You use something like a spray detailer as a lubricant, spray it in the area you're working in and lightly go over it with the clay. The clay grabs onto the stuck on things like road grime.
Easiest way to tell the difference, rub your hand over the paint on an area that hasn't been treated yet, then rub your hand over an area that's been freshly clay'd. Top surfaces will show the most.
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'11 GT Kona Blue Brembo
"Measure with micrometers, mark with a scribe, chop with an axe"
"I'm not new to driving. I've been doing it for a couple of years now." - Aphex