Hi, I have just started using Liberon sanding sealer, trying to build up a few coats (3/4) and finishing with wax. I have seen the same problems with finger nail marks. Having never used it before I am unsure of how hard it should be.
One test I have done is to apply some to some glass (side of a glass jar). So far what I have found is where it was thicker it was easily scratched off, but where the coating was noticeably thinner it was much more resistant to the finger nail.
So I am wondering if I have been applying it too thickly so it has not been able to cure correctly.