I went to Wally World and got a can of Truck Bed Liner spray. Good stuff. You can even do the bottoms and even tops of your boots. Wherever you spray it that area is protected. I spray my coil covers so they never wear out.
If you plan on never selling it and throwing it away when done, sure, spray gunk on it. My question is why your dealer didn't include a scuff cover and explain how important they are.
Scuff covers are fairly inexpensive and although i have never heard of anyone wearing out a search coil, what a cover will do is protect the coil from unattractive scratches and help maintain the resell value of your detector....spraying gunk on the coil will send the message you are trying to cover up blemishes if you decide to resell, prospective buyers like to see the bottom of the coil for some reason, i guess it's akin to looking in a horses mouth before you by it...