Since you bought a used vehicle, I’m kind of surprised you didn’t get under it before to look for stuff like this. A rental probably got washed with heavy detergent, and lots of winter salt usage if it was in the right area.
it looks like just surface rust in a spot that had some paint qc limitations...
The key to a repair like this is prep. How are you going to convert the rust? Ospho? Evaporust?
I see you want to use rust bullet, but IMO you should do some conversion first.
I hear you. It's all mentioned above though.
I inspected the truck top down and crawled underneath. These inside cab corners are in a bit of an obscure area if you don't have the luxury of a lift and like Nick1994 mentioned, this type of blasted and pitted metal really wasn't on my radar given everything else is so new under there.
Like I said, right after finding this, I scrubbed the areas (which were already pretty clean and totally flake free) with rubbing alcohol and then coated them with the rust bullet I had on hand. Per the Rust bullet link above this product preps, converts and top coats the rust all in one. Did this all before posting this thread.
If it doesn't work, I'll go the Trav route.
I'm honestly surprised no other 4th gen (2009+)Ram 1500 crew cab owner has had a look at theirs and chimed in at this point. Like I say, these inside cab corners are a serious design flaw on these trucks the way they will basically scoop up road spray from the front. Regular cabs and quad cabs have a slightly different inside corner design that won't be effected like this. Same with the new 5th gens.
I started a thread just like this one over in a busy Ram forum I frequent and it basically just got lost in the shuffle.