I don't play that game. I'd weld the seam solid and weld a full strip to close that dip or completely cut that lap joint out and replace it with a strip butt welded,use TIG welder

If you are set on keeping that lap joint original ,I can only suggest Marine-tex. I never used it on C-pillars, but have used it grafting metal to metal, fiberglass to metal,plastic to metal with amazing results,it beats the two-part metal bonding adhesive.

I'm assuming you're not applying filler over metal that was wirebrushed.