Thanks for getting back into it. Sure is something how life throws us a curve every now and again.
I think the red you're refuring to is a primer, at least the undercoat on the outside is. When I wire wheeled the rear of my 100 it was red primer, maybe the inside is a sealer of some sort. I thought I read something somewhere, that it's a rust inhibiter of sorts because the rears sat around awhile before being painted and they didn't want them rusting.
Could have just dremp that also
but I'm pretty sure that was it. I wouldn't think your going to hurt it any, but don't take my word for it, wait for one of the pro's to chime in.
But good luck, will be watching your progress.