I was wondering if this could be done. I was thinking that years ago used car dealers cut the sidewalls to make the groove for 'painted' whitewalls. Factory whitewalls have the white rubber extending out under the black rubber past the groove so it might be possible to trim a bit of the black off to make the whitewall a bit wider. I would want to try it on a scrap tire first but it could very well work if done carefully. I haven't seen one of those tire lathes for years, though.
I have also seen the sealer on bare metal so it was applied in some places before the shell was primed. I can specifically remember removing the front fenders and there was bare metal under the seam sealer. Fenders were apparently hung before priming (as well as doors and trunk lid.)
Dallas