My frame must have been a mouse house.

I found the same thing when I was blasting the underside. I scraped as much as I could and then used a pressure washer with warm water to finish it off. Blew out as much of the excess water as I could with my compressor and then just put a couple space heaters facing the areas to finish the dry up. I know a lot of folks are against using water but I figured Id bite the bulet and get it ALL out because if it got wet again when driving that mud would hold water longer and do more damage in the long run.