Can this be used over a poly finish?
Good question. First you have to sift through all the BS people are saying about what it does and what it is. Forget "feed the finish," "deep, wet look," etc. While it's not always reliable, start with the MSDS (see link). You'll see it's water, various oils, glycerin, and a very fine abrasive.
Poly, by its character, is abrasion resistant. So the effect of the abrasive will be less than harder finishes like shellac, lacquer, and auto finishes that are formulated to be abraded & polished. Note that hard and abrasion resistant are not the same thing. Rubber tires are abrasion resistant, but they are relatively soft and "rubbery." Slate is easily scratched with a screwdriver, but is quite hard.
So combine the known facts:
- Using this will add some luster, even if temporary, but the addition of oils (much like a furniture polish)
- The abrasive will have less of an effect on removing things it was designed to remove -- coarser scratches (from coarser abrasives or wear and tear).
Here is a link that might be useful: MSDS
Polyurethane, due to the shape of its molecules, are not susceptible to being rubbed and polished to a gloss successfully. The molecules "tear" rather than "cut" and the finish suffers. With poly, you get the finish you get at the time of application, and that's all. With a nice oil varnish (old fashioned, slow drying) you can rub it out to an unbelievably nice finish.