Pinch I have been in the hobby for 6 years, using an Equinox 800 for the last 3 1/2. I still dig rusty nails. Mostly by choice because of the what if ? or it could be factor. And on occasion it pays off but at the end of most hunts I have more than my share of rusty iron.
That said one thing to remember, a good target does not move, if you get a good tone in one direction, remember that exact spot under the sweet spot of your coil, turn 90 degrees and swing, if you have to move around or chase it to get a high tone more than likely junk. You can also engage the horeshoe button, grunting and minus numbers also a good indication of junk. But keep in mind, (my curse ) if I get a good tone in with the grunt I usually dig it. One thing I will say in most instances if in all metal a non ferrous target will completely override the iron sounds. One more thing I use to try to expose a rusty nail. I will do a wiggle back on the target. If as soon as you start your wiggle back you lose tone and or tone with numbers dropping substantially its junk. A good target, tone will follow you on the wiggle back to the edge of the coil. And if it does, even if you are only getting good tone and #s in one direction, dig it. Rusty iron frustrates most mdr's unless they just cherry pick the good clean signals but in amongst the iron lay many good targets too. Every signal is unique due too so many different factors so digging them is the only real way to train your ear and after awhile you will know when to dig and when to pass. I hope this helps a little. Good luck, Mark