Patents on already existing products

As already noted, a process patent or trade secret protection are your two choices. Ideas, per se, are not protectable as intellectual property (with limited exceptions under idea submission law).

I agree that trade secret protection is unworkable because your new use for the old product – which you intend to commercialize – must be disclosed to the public. So as far as the exclusionary rights available to you under intellectual property law, only a process patent could work.

The law defines a patentable process – that is, a method of doing something that has, as its end product, some tangible result – as including “a new use of a known process, machine, manufacture, composition of matter, or material." However, when the applied-for patent claim recites the “new use” of an old structure and that use is directed to a result or a property that’s inherent in the structure, then that use is not patentable. Moreover, if the new use is patentable then the patent is still only infringed by practicing the newly claimed process, not by selling the old structure. At the end of the day, new use process patents really have value only when the new use is of a chemical or biological agent. Not the types of products that you can buy at any retail store. In short, process patent protection is very likely unavailable for your new use.

If the product for which you’ve discovered a new use is not patent protected then you can offer to sell that product under your own brand name and for the new use you’ve discovered. Your intellectual property protection is under trademark law and your business advantage is being the first in the market to fill the consumer need that you’ve identified.

You should chat with an intellectual property attorney. Good luck.

Two possiblities come to mind: process patent and trade secret.

To protect your idea of how to use this product in a novel, useful and non-obvious way may be eligible for protection as a "process patent." Process patents are utility patents that apply to a particular process or method. The product itself may already be covered by patent, in which case you may want to license the patent - otherwise you may be infringing on someone else's intellectual property.
For example, if you've discovered how to transform lead into gold by heating it in Cambells's Tomato Soup, you could apply for a patent for that particular process. You couldn't claim a patent on the tomato soup, though - that recipe belongs to Cambell's.

Without knowing more about what you've discovered, I can't say whether trade secret would be a viable means of protecting your idea. If what you've discovered produces some product that you could sell, you could take measures to keep that process a trade secret, and may have specific recourse if that trade secret was misappropriated (this is governed by state law). Again, using the tomato soup example, you could set up a factory to transform lead into gold using your secret method. You'd have to take reasonable measures to keep it secret, and only share it with employees on a need-to-know basis.

If your intention is to profit from others broadly applying this discovery, then trade secret probably won't provide you any protection. For example, if you wanted to sell kits to transform lead into gold, your process would be out in the open and wouldn't be protectable as a trade secret.

Buy an hour of a patent attorney's time and find out the best path. You'll be able to speak confidentially and find out if you've got the next million dollar idea on your hands. Good luck!