Well this should be very interesting indeed if you guys and gals take Ron up on it.
I won't be one of those taking pictures and competing here
for one reason over all others, I have never had a customer complain about "shadows". I get hired to clean the sidewalks, patio areas etc. and I do that well.
They are informed that the gum comes off and it leaves that "shadow" which over time tends to lighten, in reality it is there and always will be like Ron said.
I never tell them I can remove that "shadow" as I know it will not truly come out. Yes many have claimed to make it go away, some cheat and sprinkle concrete powder down then wet it, great white wash job for a few month but it goes away too.
Now Ron, the key question here is just how are you going to require proof that the before and after pictures are of the exact same spot?
I will enjoy watching all of you claim you have the best or only method or that you have developed a product to do this job.
Pulling up a comfy chair and sitting on the sidelines watching all the excitment here, gee what fun
Could even ante up a buck each for the winner.