Oh well if you are going that far, and you wil have to repaint quarters and sills I would scratch the surface as you will need it for the filler to stick. P40 filler to start with, to give you the build, then p38 for a finish.
Or if you are just talking about a bead along the edge so there is no gap, the likes of the window bond would be fine. Smooth it along with a wet finger and use panel wipe/white spirits, not thinners, to remove excess.