Anybody can use what ever they think is best for their tires, really...I'm also thoroughly amused by the invectives constantly thrown at Coker.
They don't
make the tires, they are made by Michelin for them in Michelin's Mexico plant, in Queretaro (Bajío Region). They were made by consultation with the German Mercedes Old-Timers Club.
Coker whitewall cleaner does NOT "require" sandpaper. It was merely a
suggestion when I could not get mine as white as they were when new. If you have not seen 1000 grit sandpaper, take step back and look at it first; it's on par with a brillo pad or soft scrub which others suggest here. If one feels it is too abrasive, similar papers are available in finer grades--1200, 1500, even 2000. You can always substitute a non-metallic scrubby pad or sanding sponge. I have not found it difficult to control--not at all. I just use small pieces and I use it wet with the cleaner. This isn't my own invention, it was suggested to me when I called Coker.
It's great to complain about current white walls, and how the old whitewalls didn't ever yellow. Whatever--but what we have today is what we have to deal with...like ethanol in fuel! The "good old days" are gone.
There is a big difference between tires of yesterday or ones on a daily driver, and tires we keep on a collector car. I put 25,000+ miles annually on my daily driver, and at that rate (if I was buying tires, but they are management lease vehicles) I would be buying new tires every two to three years, simply because the tread would be gone. In a collector car such as we have we are keeping the tires for a lot longer, thus some issues such as ozone cracking, leaching of carbon black into the white walls, etc. start to show up whereas you'd never (or rarely) see this on a tire that wears out in 2.5 years. I think my Cokers are about 9 years old now--and maybe not as white as a DiamondBack, or new Cokers, they still are pretty good. The super cleaning and white treatment that I do is not a regular process, only before a show or maybe once a season beyond that. My WW still looked great at 2 years!
The "bottom line" is doing what you want to do. If one feels that Bleche White is the god send for your WW tires, or Soft Scrub, or a Brillo pad and Comet/Ajax, well then by all means use it. If both Diamond Back, and Coker don't recommend products containing bleach, then that is something to pay attention to. Personally, IMHO, if Bill and Tab Chapman of Diamond Back--who make their living on WW tires, make a suggestion: I'm going to listen.
In Diamond Back's FAQ on "How Not to Clean Whitewalls" comes this:
And speaking of ruining whitewall tires, right here is the perfect place to mention how NOT to clean your whitewalls. If you want to ruin them over time, one of the best ways is to use cleaners that contain bleach. Yep...whitewall cleaner. My guess is he is speaking to Soft Scrub; Comet, Ajax, etc. (For its invasive name, Bleche-White doesn't actually contain bleach...it's multiple organic solvents and surfactants. It's the solvents in Bleche White that have long term effects on the rubber.) I don't make this stuff up, it's from the experts.
BTW, I've been interested if not intriqued by all the good reports on Vredestein Sprint Classics...when the time comes, I'll buy a set of those and have them treated to WW by Diamond Back!
Seems that the Diamond Back guy has figured out that a layered system of WW rubber is better than the conventional single layer. I guess you should expect expertise from an expert?