Did you notice that the F-70 when it hits a can will jump all over the place with number readings. (78 to 90) ?
On clad quarters, for the most part it will hit 81 and stay there. On a silver quarter, a solid 85. On dimes, anywhere between 70 to 74...no matter if clad or silver.
On nickles, 31 solid. The problem with finding nickles is that certain tabs will read close to nickles. When I hit a nickle reading, I swing at every angle and if it jumps 1 or two numbers up or down, I know it is a tab. I've gotten to be about 95% accurate in predicting nickles or tabs.
Today, I hit a quarter down 12 inches using autotune, slow speed, sensitivity to 72 and threshold to -4. I also tried 0 discrimination and it read it just fine. That quarter made me mad because it was a 1986 clad and was under a root. I thought that quarter was old because it was so deep and covered with a large root and I had taken several standing Liberty quarters there. It took me about 15 minutes to dig it out. Not that I hate clads but spending 15 minutes to make $.25 is not good time management.
You are right that the F-70 is very light. I can go out and swing it for 8 hours at a crack. Did that this weekend and found $44.23 worth of clads at a local soccer field / park / playground. Also found a gold wedding band and a silver ring. The gold ring at 6 inches and the silver ring at 9 inches. Not too shabby.
Coins deep do not necessarily mean old. I have found pre 65 coins at 2 inches and in the same area, clads at 6 inches. Go figure.