One thing to keep in mind.
You don't really remove the scratch, you bring the paint around it down to the same level.
Slight marring needs very little paint removal. A deep scratch may have you removing enough paint to compromise the clearcoat.
Deep scratches that catch a fingernail usually require paint to fix.
The Langka process has been a mixed bag for me. Sometimes I was happy with the results, other times I didn't have that good of luck. As with most things, practice would probably help. One nice thing with using the Langka if you don't like the results, you can remove the touchup paint with the Langka and start over.