The flux in the solder leaves “unpretty” marks on the pcb. (see below). Those residues can be cleaned with PCB-cleaners (like: Kontakt LR).
Subsequently the PCB-cleaner residues need to evaporate. Give it 10 to 20 minutes.
ATTENTION: Clean the board under no circumstances with the pots already installed, they can be damaged by the pcb-cleaner.