Not sure if there is anything you can do. Most rats insist on scent marking. My boys are pretty much litter box trained and don't urinate outside the cage, however, they still insist on peeing on their fleece. I figure I can't be but so picky about it, I know that it's their natural tendency to do so. Not sure if anyone else would be able to give you more info, as far as I know, most other members have the same thing going on.

I think it's something you'll need to learn to live with. It's the reason I put down liners, so that I don't have to wipe pee off the shelves. I can just pull out a liner and put in a new one. Unfortunately our ratties are little piggies. 

Yep! It's like getting dogs to only pee on only one part of the fire hydrant (supposed to be a joke but doesn't come across that way in text without an explanation...heh). They're not typically "potty trainable" when it comes to urine.