I think this is sweat related, you should be able to use a rust killer or converter on the rust then use touch up paint. My 81 Bianchi SC has this problem to the point that one of the cable guides is rusted thru. I have used a little black paint to touch these up as this is the way they were original to the bike. Cable housing would collect moisture underneath (sweat being really corrosive) Oscar Juner would always say lots of coats of wax were always a good idea.
BOB FREITAS
MILL VALLEY,CA.....USA
were the long weekend has
been spectacular