The 25-year-old put his local knowledge and superior Fiesta Rally3 experience to full use, extending his lead in the all-Ford category.
He won all eight stages despite nuring a minor technical issue and headed Jan Černý by 3min 34.7sec approaching the rally’s final day.
"We had some small technical issues, but we managed to deal fine with them," Joona explained. "It was not a big problem, but when your steering wheel is a little bit to the left while you are going straight it can be quite distracting.
"I am trying to compare my times with the WRC2 cars and I think it was a successful day for us, but we can still go a little bit faster," he added.
Henri Timonen completed the podium 12min 34.3sec further back, while Finnish teenager Toni Herranen plummeted from second to fourth overall when he hit a rock in the second stage of the day.
The impact damaged the car's rear left brake caliper and he dropped over 15 minutes while limping to service with limited stopping power.
- Full coverage from Secto Rally Finland is available on WRC+ All Live here, including every stage broadcast as it happens as well as key interviews, features and expert analysis from the service park.