Haven't read all this thread through so someones probably already said this, buts here's my process:
This brings my tubes up as new everytime (the boat is kept mainly under cover though)
Wash with a detergent cleaner such as Star Brite:
Dry off with leather, then apply (out of direct sun if your rib is anything but grey or white) Autoglym Super Resin polish - This removes just about anything (apart than deep rust stains ive found, but it will lighten these too), buff off and apply again if need be - then:
Seal with either 3M tuber conditioner or Autoglym Vinyl and Rubber care - both of which will make the tubes slippery, so I then wash the boat off with cold water and leather off once more.
Although Super resin is slightly more £ than other similar stuff you can buy in halfords, the finish it leaves, even on rib tubes or fiberglass is superior.
Sunseeker (used to and as far as i'm aware, still do) buff with Autogylm metal polish on all their hulls and GRP surfaces out of the mould, they say nothing gets a finish like it (and they always have the best finish at the shows!).