Since it is a water damage which not covered by guarantee, maybe it is a good idea to go to a local repair shop. You can find a list of Fairphone friendly shops here:
I had a water damage on a tablet PC last year (never ever put a water bottle into a backpack with your tech stuff…). I gave it directly to a repair shop without turning it back on. They opened the device and cleaned the logic board professionally. Even after one day there was already some corrosion on the board du to the liquid - at least that’s what they said. Despite some discolorations on the screen everything else is now back to normal.