I have also tried the Dr Colorchip and found similar results to the Langka - its very tough to get the paint removed just enough to stay flat without removing too much. Especially when you have light scratches that don't hold the paint well.
That said, the Dr Colorchip paint is nice and thin and dries within minutes, which is nice.
I am thinking that painting over a thin scuff or scratch, and then wetsanding to make it smooth, might be a better and cheaper option.
I'll keep trying the Dr Colorchip to see if I can improve my technique.