Plus one for Waxoyl, I remember spending a lot of time treating the edges of doors and tailgates with the stuff, back when men were men and the sort of cars that I could afford were made of any old rubbish that would rot as soon as you turned your back on it (especially Ford Capri tailgates). I had a grease gun like applicator which screwed into the can, for injecting it into nooks and crannies.
It takes a bit of mixing, I still have some in a can, which will have split into spirit and wax by now.
Having seen this thread I am minded to have a go at my CL450 guards with it, as they won't be making any more of them anytime soon.
My old 911 has some notorious hidden rust traps, after advice from someone I was talking to at a car show, I painted all the hidden underpinnings with a liquid rubber product intended for flat roofs. Maybe that would work under mudguards too.