Oof, that's rough. A heat gun on a panel for that long is way too much direct heat, you'll cook the paint every time. For cold metal, you gotta use a torch with a wide, soft flame and keep it moving constantly, never letting it sit in one spot. Just warm the metal enough to take the chill off, you don't want it hot to the touch.
Oh yeah, "take the chill off" is exactly it. I did the same thing on an old F-150 door in January, got impatient and held it steady for like 15 seconds too long, ended up with a hot spot that turned the paint cloudy. Now I just gently wave the torch back and forth until the metal feels slightly warm to the back of my hand, not hot at all. Works way better, less risk of ruining the paint.