My '79 W150 is from out west and APPEARS rust free, but from this thread, I'm concerned about what lurks beneath.

I wonder if I would put some rust converter in my pump sprayer (I realize it may ruin my sprayer), and after removing the visor mounts, spray the best I can all up in the inner roof panel to kill any rust that could be lurking under there? Maybe I can spray through the dome light too if I remove that.