[Zedn]

If the movement is going to be that much of a problem, then it will extend to the paint cracking as well. I don't know if there is a definite answer. I can only think your choices are plain bog, fibreglass, filler with glass strands (hair) or something really smooth like a skim putty. All will crack with excessive movement. I guess you just want to keep the sikaflex as thin as possible and make sure it is fully cured before filling to allow for shrinkage. I would probably use the filler with glass strands (kahglass) then skim it with something like dolphin glaze (upol).

Just my opinion. That's what I did for my doors when I did them. Glued skin to frame and filled the edges to the inner panel.

Do some searching on how they install body kits on the jap imports. I think a lot of those are glued then smoothed in with filler.