The flux in the solder leaves “unpretty” marks on the PCB. (see below). Those residues should be cleaned with a PCB-cleaner (like: Kontakt LR or Isopropanol).
Subsequently the PCB-cleaner residues need to evaporate. Give it 20 minutes.
ATTENTION: Clean the board under no circumstances with the pots already installed, they can be damaged by the PCB-cleaner.