por 15

Yes, the manufacturer claims it works well on flexible parts (I haven't personally tried leaf springs yet). Just be sure everything is perfectly dry between the leafs (after cleaning and etching) before applying the Por-15.
 
I did the leaf springs on my truck just to see if it would hold up. So far it has, its been two years since I did that. probably have 10,000 miles on the truck.

I did not do the entire leaf spring, just part of the bow where it flexes a bunch. It did fade quite a bit. because it was in direct sunlight.