I'm not Finnish but I've always wanted to visit and plan to do so for 6 months. While I'm there, I'd like to volunteer through WWOOF.
Is this even possible? I can't find information about visas for this type of situation in particular.
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Finland is part of the Schengen area, thus generally the same visa rules apply as with any other Schengen country.
With a Schengen visa you can stay in the Schengen area for up to 90 days in any 180-day period. Some Schengen countries issue national long-stay visas but Finland does not.
The police may extend the visa validity under exceptional circumstances but this does not apply to this situation.
Thus, it is not possible to legally stay 6 consecutive months with just a visa.
You need a residence permit unless you are EU, EEA or Swiss citizen or a family member of a EU, EEA or Swiss citizen.
BTW, when asking visa questions you should always state your citizenship, as many countries have special bi-lateral agreements.