chudford wrote:
I have a windowsill in a bay window made of 3 sections of wood.

The joints between the sections need to be filled with something.

I have tried polyfilla and various flexible fillers but the joints
keep cracking.

Any ideas for solving this problem?

Thanks
Wood moves. You won't stop it. It's nature at work.
Use a suitable shaped wooden moulding and only fix to one side of the
joint. Paint underneath first so when the joint moves painted wood shows.
Bob