This is an extremely interesting conversation, seeing that there is one going on over at the Great Lakes Angler website, since last year. I am a big salmon fisherman, I try to get back to WI a few times a year every year for salmon fishing. The WD40 product and the fool-a-fish product have been verified by several of the fisherman there, that I know and trust. These are the comments/facts as I know and have used them.
WD40 - Is a proven tournament technique, it does not place a scent on the lure though, it removes YOUR scent. It does have to be reapplied everytime you bring the bait in and after you are done touching it. The downside is, its a chemical product and is bad for the environment.
Fool-A-Fish spray - Is not a fish scent spray, but a "visual" spray. The best way to use it, is to spray it on your lures the night before to dry and it will last several hours underwater. If you use it the day of, then it will wash off pretty rapidly in the water. Fish see in 4 levels of the light spectrum, we see 3 (Red, green, blue). The fourth is the UV/A light spectrum. The spray is used to coat the lure and reflect this spectrum underwater. The thinking goes that UV penetrates the water deeper/farther than the visual spectrum range does. This product has been proven in fishing tournaments and on days when the fish just aren't biting. It is not that more significant but when you are catching an average of 1 more fish out of every 3, it makes it worth it IMHO.
Take this for what its worth, but salmon and walleye fisherman are finding these two products an integral part of the tackle box, one more weapon to use.
Kinda funny we need such ingenius methods to fake an "animal" with a brain the size of a chickpea.
Forgot one thing: To see the Fool-a-Fish spray, take a blacklight and illimunate the lure w/the spray on to see the effects of it.
