[m22mike]

I read a article on trailer tires last year, and in the article it was noted that there are paraffin's and chemicals in the tire compound that comes to the surface with use.
I would have to believe that after some time your problem will improve.
I think you have done OK with the Bleach White. About once a year I wipe my tires down with mineral sprits, it really cleans up the grim and oxidation,STP Son of a gun is all I then use , I do not like Shinny. I would use whatever dressing you like.