We did 2 things. Firstly tried to fill the gaps with spakfilla and then paint over the top. Didn't look too bad. Another section we didn't worry and just painted over the top, which I think looks better because it's 'cleaner' I.e
doesn't look like we've tried to hide the panelling. If you go with the spakfilla option, do small area at a time and finish off well. And sand back.