As th'r said, you should not be digging ANY nails (will get a null/reject on them), when in disc. mode, even at the lowest settings. In order to knock out foil wads, you can increase the disc. knob clockwise to where they start to null out as well. To determine the point on the dial, you can use some test wads, and do air tests. Slowly increase the knob, till you null them out. You'll notice that the bigger/heavier the wad, the more increase in the knob it takes to tune it out. So that to the point where, ......... if you had a wad of foil as big as your fist, you'd probably have a wad that reads as high as dime or quarter, and thus not be able to disc. it out. Fortunately most foil wads are never bigger than a peanut or gum wrapper size
And as th'r says: if you knock out foil and tabs, be prepared to loose gold rings too. But sure, in junky blighted parks, it does NOT pay to be a "hero" and try to strip-mine out the foil and tabs anyhow. If someone were that h*ll-bent on finding gold rings, then junky turfed parks are not the place to be hunting, to begin with. Might as well just go to a swimming beach, where the ratios are better and the digging is easier, if gold rings were your goal. In turf, I usually angle for the old coins only.