I am preparing to Sadolin my window frames - the sills (Cills) on the front
get most of the sun and have some quite bad lengths where the grain has
opened up - beyond the point where I can hope that a thick layer of Sadolin
will fill the cracks. Clearly I need to do something otherwise water will
get in and freeze in the winter and cause further problems. The woodwork is
20 years old - but I hope to get a bit more life out of it.

Can anyone recommend a filler - ideally in a very dark colour - that is
suitable for these quite thin cracks (1mm - 2mm)