I'm confused with the answers from db7 and ozzie. I have used the Finnair Long Haul Lounge earlier this year with my OW-sapphire status (AA PLT) and economy ticket. Well, maybe it was a mistake...below are all Finnair lounges at HEL (from AY website)

HELSINKI

International Terminal - Non Schengen

Finnair Long Haul Lounge
P, CC (long haul)
Finnair Plus Platinum members and business class customers on Finnair long-haul flights.
Transit hall, between gates 30 and 31.

Gateway Fresh lounge
P, G, S, FC, CC
Non-Schengen terminal,
2nd floor, next to coffee shop

International Terminal - Schengen

***) Golden Gate Lounge
P, G
Regardless of travel class or airline.
Transit hall, second floor, opposite gates 28 and 29.
Sat - Sun closed

Gateway Lounge
P, G, S, FC, CC
Transit hall, opposite to gates 28 and 29, 2nd floor.
Open Mon-Fri, Sun 06.00 - 19.30, Sat. 06.00 - 17.00


Silver Wings Lounge
P, G, S, FC
Finnair Plus Platinum and Gold members regardless of airline. Finnair Plus Silver and Finnair Club members when departing on Finnair marketed flights and Finnair code-share flights with British Airways, regardless of travel class.
Second floor, above gates 26 and 26A.

Gateway Tower Lounge
P, G, S, FC, CC
Second floor, gate 25.
Open Mon-Fri 06.00-21.30, Sat 06.00-19.00, Sun 06.00-20.30