So, I decided to give my bike a thorough cleaning and relube last weekend, and everything went fine except for one thing...
It appears that over the last year or so that either my sweat or road salt has made corrosion marks on my cranks. These splotches won't buff out with a t-shirt rag, and now I am starting to think about other treatments.
So basically, I want to ask if anyone has any ideas.
I don't think this is totally an appearance thing, its also about safety-- it is really important to be able to periodically inspect the cranks for cracks, and now with these whitish spotches that is getting hard to do.
I think that this is totally about appearance. Not meaning that your concern is totally about appearance, but that white splotches and corrosion marks (either from road salt, or perhaps from your shoes buffing the cosmetic anodizing layer on your cranks) do not pose a structural danger to the cranks. Aluminum does corrode, but most corrosions of aluminum form a layer that resists further corrosion - a very nice feature.
So, you don't need to worry about this as a structural concern.
If you want to buff them off, a scotch-brite pad will do the trick.
Any metal polish or even car paint polish (the kind that removes tarnish, not the wax stuff) will do to polish up the aluminum, it won't last very long as aluminum tarnishes easily and marks easily. You want to be sure the surface is really raw aluminum and not anodizing or paint. Raw aluminum is uncommon these days and some sort of surface treatment, even if only clear coat over the logo, is common. Polish will still clean these up, but you run the risk of removing or thinning any coating. You really have to rub hard to do this, compared to removal of superficial dirt/ grunge but keep it in mind and test on an inconspicuous area first. Scotchbrite pads are pretty abrasive by comparison and will leave a slight scratched satin or dull finish.
Buff them with the shirt and forget about it for a month. You can go nuts with ammonia, polishing compunds, etc..., but if this is a commuter or around town bike, you're just asking for more frustration. You'd need some ebola corrosion for it to do any mechanical damage. The dark spots are actually chemically the same as the anodization that is used as a "finish" to protect aluminum; the only stuff that you want to get rid of is the white, powdery corrosion.
Mother's Billet from any auto store! Just follow the instructions, and do use a very soft cloth for the end step. A dollar store near my house sells the really nice fine cloths for $1, so I don't feel bad getting it all black.
You'll be very impressed! And the product also protects. My aluminum cranks now have a mirror finish that's lated three months!
I cleaned a 15 year old Alu Rocky Mountain that went from mat grey to super shiny. I could see my reflection in the welds.
There's only one danger, you'll want to polish all your components, and friends' bikes, and...