Wash as much debris off first for best results and try to keep a clean foil surface available. Been doing this for years and love the results.
And, for what it is worth, foil can be used to polish many surfaces, such as bottom bracket bearing races. Not sure if there are any negative impacts associated with this practice but I have been doing it for years. And not one bearing failure to date. So, at least it does no harm.
This is not a good idea to use on pitted races even though it will make them feel smoother to the touch.