I sprinkled baking soda on a pony, and then sprayed it with white vinegar I had in a spray bottle, and rubbed a bit.
Now it looks like the cancer/age spots are much lighter on both ponies!
Not gone, but faded till they are hard to see. One pony was a chief, and the spots are a light tan, and on my baby Hopscotch, I have to look for the spots now to find them, they are just a few shades different then her body color.
I'm not sure if my eyes are just tricking me, though, so I don't want to get anyone's hopes up, but it might be worth a try?