Stopping rust inside door panel

[MASTERBrian]

While working on getting everything lubed and working properly on my door, I noticed what resembles a bunch of rock wool in the bottom of my door(s). Someone told me it might be what they originally sprayed the inside of doors with to somewhat sound deaden the door. It's that our the work if pack rats, but never the less, it was damp and needed to come out, which it did.

Now I'm tasked with, how do I treat the insides of my doors so they don't rust any more than already probably are? Spray rust neutralizer in there and hope for the best? I also have a jug of metal etch.....is that a good solution?

If I can get satisfactory results from one of those or something else, what's a good way to coat the inside to protect from future? Spray it with primer and paint and hope for best once again? I'd also like to deaden the panels so what about sound deadening spray? Keep in mind I don't have much access and I don't want to cut the door open.

This is on a 1960 door.