Hi and welcome to the forum!
We are in the middle of a window fixing project at the moment, luckily enough we could hire a traditionally oriented carpenter to do most of the job. (Two small kids, one felt roof already made by ourselves this summer => total exhaustion...) Our guy uses Uula's paints and his choice was linseed oil paint for the exterior frames and Uuula's Ovi-ja ikkunamaali (Door and window paint) for the "doubles" ("tuplat") as the inside frames are usually called. You shouldn't use Ovi- ja ikkunamaali for the exterior ones since it is not suitable for exterior painting, but it dries about ten times faster than the real linseed oil paint, so it's much easier and faster to paint the inside frames with it. There are other manufacturers of the traditional paints as well as Petteri mentioned, Uula is just one example - but they have translated their website in English as well,
http://www.uula.fi/en/.
I wouldn't use any modern, water-based paints - they make thick plastic cover on top of the wood, which can cause trouble especially on the walls: paint eventually starts to flake in big pieces and looks really ugly, and in the worst case the wrong kind of paint can destroy the original panels or window frames because the plastic layer prevents the wood from "breathig", e.g. absorbing and emitting moisture. You can pretty easily see the difference between oil paint and water-based paint: traditional oil paint looks like a crocodile skin when it gets older and if you scratch a bit of the paint and bend it, it cracs in small, sharp pieces. A piece of water-based paint, however, feels and looks like a piece of a thick plastic bag, I would say... Linseed oil paint is glossy when the surface is newly painted but it looses its gloss over time.
With linseed oil paint you must paint very thin layers and actually almost rub the paint on the surface, as Petteri said. And it really takes a long time to dry!
By the way, if you use white paint for the frames, as I understood, it can be tricky to find the right shade of white. Plain white without any shading can look weird . We did it the easy way - the carpenter told us that he has some amount of white paint left from his previous window project and we decided to use that
It was a good choice anyway and looks nice, if I remember right the shade was named Pouta.
"Saavat tehdä tämän päivän talot juuri niin kuin haluavat, mutta minun talo on tehty kuusikymmentä vuotta sitten ja semmonen aika näkyy talossa ja ihmisessä. Kiitos mutta ei kiitos." - Tuomas Kyrö: Mielensäpahoittaja
