Drivers Fernando Alonso and Lance Stroll were reckoned to have lost 0.2s on the main straight in Baku during the Friday practice run and both the grand prix and sprint race qualifying sessions.
The malfunction was primarily caused by an aerodynamic characteristic of the new low-drag wing specification, which meant the overtaking device was oscillating and would only intermittently activate.
This was exacerbated by the coastline street track's bumps and air pressure on the long straight.
By way of a temporary solution for the Azerbaijan GP, in which Alonso finished fourth and Stroll seventh, the team cleaned the rear wing endplates with lubricant to ease the movement of the flap.
Asked by Autosport to explain the quick fix, Krack said: "I cannot tell you, but it was not far from using WD40!"
"The car is in parc ferme so there is not so much you can do. We tried to clean the surfaces and tried to check your gaps, everything.
"We checked everything with the FIA and the FIA was very helpful on this and you will have seen also we changed one flap on Fernando's car.
"This was after a routine check, basically, because if you pull all the time on the flap when it is not moving then you put more stress on it. So, for safety we wanted to change it."
Source: Autosport