I have an old farm house I just bought. The exterior is lapped siding. The bottom piece of wood (don't know the name) hangs out about 1/2" over the foundation and serves some sort of drip purpose. Well, this piece of wood is really weathered and has very large and deep cracks in it from age, sun, and other weather.
The paint is falling off of it on the south side and the wood is fully exposed.
I have never had great success filling cracks or holes in outside trim with wood fillers or even epoxy fillers long term. It might be easier to replace the wood or maybe try caulking if it is only cosmetic.