I chased some paint bubbling at the lower rear of my driver's door and sure enough, it was from rusting covered by filler. But after looking at the inside (took panel off) it looks like it is designed to capture a certain amount of water before it gets to the 'overflow' cutout at the center. What's the best way to handle this? I'm guessing taking it all down to the metal (ugh), cutout rust/weld in new metal as needed, then use seam sealer down there on the inside before painting. Is that the way to go?

40 Chevy 1/2 ton