Full Tones will allow you to hear the wavering sound of foil and can slaw. Try to determine size and shape of the target. Foil needs to be large size to put out a strong signal. Learn what a ring-sized target signal looks/sounds like. Foil not round shape and has to be bigger in size to get the same signal strength. Lift coil off the foil target and signal should drop out very rapidly. Try increasing the sweep speed and it should breakup. Foil is usually not deep. Get practice with the XY screen and you can usually see a bad target. But dig all until you have enough practice. You can have a foil check program set up that you can quickly switch over to to double check. If you are in a foil area you probably need to raise the disc level. 42 sounds bad but you know, thats why they have discrimination. Or you can put a notch in that area. You need to find that cut-off point and it will change depending on the area. Small can slaw you probably just have to dig it.