jdeiana,
I am a relic hunter and not a coin hunter and nails are a definite problem. In my experience, old rusty cut nails will frequently give a good signal. If the nail is bent or curved it can give a perfect, repeatable 32-34 with a high tone that sounds just like a bullet. Generally though, the signal changes as you rotate your swing to a different angle. Increased soil mineralization seems to increase the false readings.
Another rusty nail phenominon that I have seen is the "moving target" syndrome. You get an iffy target, pinpoint and start digging with the target in the center of your hole and once you get down 8-10 inches the signal suddenly moves to the edge of the hole!. Widen the hole and the signal continues to move to the edge until you finally unearth a very rusted cut nail at 6 to 8 inches from where you pinpointed! A good bud with an F-75 has seen the same exact thing happen numerous times and we now have a theory. We have noticed that when this happens, the initial false signal is frequently downhill from the actual target. Could the rust be causing the halo effect and, as gravity causes the rust to slowly flow downhill the edge of the target moves away from the actual target? I had noticed that on large rusty items the target is at the edge of the item and not in the center. Example: I recently dug a 14 inch piece of a plow disk that was laying perfectly horizontal, about 6 inches down. When I passed over the item I got a small sized but very loud signal (32, good tone) and started to dig. The target was the very end of the fragment, I dug and started prying it out of the ground and realized that it was more than a foot long, but only the tip was giving me a signal?!?!?!?
I am running my Safari in manual sensitivity and usually set the level at a couple of notches above "stable", basically replicating the ETrac "Auto +2 or +3" setting. I have actually had targets that turned out to be the rusty remains of a long ago dissolved piece of iron. When you dig the plug, the target is there, but when you break the plug into smaller pieces it goes away. Put the pieces back together and the target returns, and if you carefully examine the plug you can find the rusty stain left by the now missing piece of iron.
At any rate, my point is that rusty nails can give you a false promise, but if you don't dig them you will never know what you left in the ground. I would rather have my machine sensitive enough to sometimes fool me with a piece of junk rather than have the machine not sensitive enough and miss the deep targets.
Just a humble opinion,
TomH