My '89 extended cab F250 has rust on both sides of the cab right behind the front door opening at the bottom corner. It's an '89 so the extended cab does not have a separate door. I couldn't figure out why it was rusting so quickly in the same exact spot on both sides. One day as I was watching the rain I noticed it really dripping frequently from this spot. Even after the rain stopped I have noticed it dripping from this spot for an hour or so after everything else has dried. Clearly it's holding moisture. Today I took a peak underneath and found the stuff in the picture below. It looks like chewing gum lol. But it's hard as a rock and is actually painted like the rest of the metal around it. Wouldn't I be better off knocking that stuff out of there so it won't hold moisture anymore? I plan on sandblasting that corner and seam and don't want to have rust back immediately. Does anyone have a picture of that

spot on their extended cab? If I knock that stuff out I'll end up having maybe a 1/4" opening between the sheet metal. I don't think it is providing any structural support. Any ideas/suggestions?? It's started rusting very quickly since I picked the truck up a month or so ago.

 

There should be drain holes in the bottom of the door...it they are blocked (which is not uncommon), that's a very likely source.