Possible first summer Baltic Gull (Larus fuscus fuscus), but an advanced intermedius cannot be safely ruled out.
The feather appearance and the moult progression indicates a first summer Baltic Gull (Larus fuscus fuscus). There's only one problem with the identification: it isn't ringed and therefore the true origin cannot be safely determined.
Besides the advanced body and primary moult the overall darkness of the upperparts indicates fuscus. Mantle, scapulars and parts of coverts are mostly renewed to an adult-like blackish type with a slight brownish tinge and some pale edges, but some coverts are also of the browner type which has black fuscus-like shaft-streaks (not anchor-like pattern as in graellsii/intermedius). Tertials are blackish with broad white tips, underwings have mostly clean white areas, head and underbody are also almost clean white whereas belly is all clean white. Tail has rather extensive black tail band and seems rather fresh and the bill is bicolored fleshy with blackish subterminal markings and cutting edges.
The outer most primaries are worn juvenile feathers. Naturally a more advanced primary moult would have supported fuscus ID far more, but some fuscus appear to retain the outer most primaries throughout their first summer. One could also claim that the proportions does not cry out for a classic fuscus which is slim, slender and highly elongated, but judging from photos proportions does seem vary quite some in Finland.
Other rather similar looking birds are here, here, here and here