Again, as above if you want it gone forever you need to remove the rust.

But.....you will NEVER completely remove rust from a panel, because of the way rust forms. The best way where possible is to ditch the whole panel and replace it with a new one, but obviously on the sills thats not very easy.

Cutting the rust out will considerably prolong the life of the panel, but it will always come back, maybe not for many years though if done to a hgih standard