My sister has just moved into a house in northern Sweden, made of
wood. Now that she has painted it a lighter colour, gaps are looking
uglier between the top strips of wood on the outside, and the timbers
they are nailed onto. (The strips are about 4-5 cm wide and 1-1.5 cm
thick, and there is about a 4-5 cm gap between them).

Given that the house has stood there since the 1930s, and has been
regularly repainted, I doubt that there is any structural problem. But
the gaps don't look very nice. In many places they are much too deep
(e.g. 3-4 mm) to fill up with paint.

I should add that the gaps aren't evident all over the place - just
under about maybe 10% of the strips.

What filler should we use? Something with a bit of flexibility, as
used between windows and masonry? Or common-or-garden builders' caulk?
Or what??

Cheers,

John