Actually, in dyeing traps the dye is reactive with the rust (iron oxide) and forms a dark compound. That's why you can't dye galvanized metal, no matter how rough it is. There is no iron oxide for the dye to react with. There is no adhesion factor in the dyeing process.
Dipping is another matter entirely. Dipping is akin to painting. There is no reaction factor between the trap and the dip. The dip has to "stick" to the surface of the trap. If the trap is slick, shiny, and smooth, it will be harder for the dip to adhere.
That said, there are a number of trappers who don't put anything on their bodygrip traps. A light surface coat of rust can be beneficial in helping dip to stick, but any rusting beyond this point will be detrimental to the trap. Primarily, it will weaken the springs. I use dip on my bodygrips because it helps to protect them from further rusting. It is not as slippery as wax, which I use on my smaller foothold traps.
So, the quick answer to your question is: We put a protective coating on bodygrips to keep them from rusting further.
But here is a personal note. I never "pre-rust" traps that I'm going to dip. I boil the grease off them and dip them. I do notice the dip flakes off significantly the first season, but then those spots rust a little, and the dip sticks better the next time.
Hal
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Endeavor to persevere.