Yep, we have two neutered male cats that we have had this experience with. Our indoor only cats tend to do this primarily when a kitty "interloper" is seen in "their" yard. One of them also suffers from chronic urinary tract problems for which he is being treated with medication.

We use "Nature's Miracle" and another deodorizing product when they spray. There is another product, but I forget the name, that is supposed to mark the area with "happy scent" (like the scent from their whisker area when they rub on a person or object which says "you are mine."). This "happy scent" is supposed to keep them from marking the area with a "distressed" scent.

If you have taken Montanna to the vet and he is well, you might try keeping him inside only and use the products above. If he doesn't smell other interlopers, he may stop marking. Also, you might try a covered litter box to keep the spray in--and keep the box sparkling clean. We haven't had a problem for quite a while.