I would try upping reactivity as Calabash and Darktower mentioned before notching wraparound (and in that case I would ONLY use notch at 12 khz (unless you are using ID Norm in which case I would not notch at all) because at 18 khz or 12 khz with ID Norm you might miss some legit high conductors by notching high) and/or also lowering sensitivity as Ted mentioned a la the sifter. I would be interested in what Tedyoh has to say about this, but I am not a fan of running sifter outright in these conditions because Gary recommends full tones and 0-1 disc and I fear that leaves you susceptible to iron down averaging which is why disc (10) + pitch is recommended for unmasking.
Well I was just kinda of the top of my head responding to the OP's question, on using the LF coil and 18 khZ, read into it too much i suppose, like if no other options (frequency) were possible, and assumed we were talking about an old home site of foundation....
Before I had the HF coil, Gary's Sifter program was the last program I'd use at a site before I felt I "got most of the targets" - I hunt for relics, at no to very low modern trash sites, so for me that means I will dig anything that comes in, sounds better than iron - give me a clean signal reading at 25 and I'm digging, knowing it's probably a shard of brass or tin, I don't care, so the high conductors getting drug down by iron theory I never really think about as I just about "dig it all". To me the low TX and low sensitivity settings work great together to help unmask in super heavy iron (nail beds) by not overloading the coil and machine with all that rusty ferrous the coil is seeing. I tried CD's pitch program with the LF coil and didn't find anything I missed.
Enter the HF coil - the last chance I had to detect was the week after thanksgiving before deer season in PA for a few hours - went to one of my favorite old relic home sites armed with the HF coil, CD's Pitch Program in the backpack and the rifle slung over my shoulder, (this dual outdoor adventure worked out better than I though, detected from 1pm to 3pm then deer hunted 'till dark) only went over a small 50' x 50' area, some keepers were 2 shallow fatty indians, few suspender clips, fork and spoon parts, a deep sleigh bell and a very cool old lever lock, oldest lock I've found (I knew this was big iron) - plus I was battling high dead weeds so I can't wait 'till spring...it's about an acre site, looking at it as virgin again due to CD's Pitch Program and the HF coil
CD's Pitch Program along with the HF coil is finding stuff I missed with the LF coil - CD's pitch program is now my last program I will be using for that "I got about all of the targets at this site" feeling.....I too may lower the disc. down to 5 or so though.