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

Bay sills are a pain because, for each joint, you have two sections of
end grain sticking out in the weather. They're often well rotten inside
before anything shows on the surface.
Anything flexible enough to withstand the movement is likely to pick up
grime and look a mess.