Eastwood sells a zinc phosphate spray can product. It is too dark for most things on the cars IMO except it approximates the dark heat treated steel appearance that the leaf springs look like when fresh. It is the closest thing in a already mixed up formula for the leaf springs I have found if you are going that route.
Hi Bob are you referring to this stuff? I had a can kicking around that I never used before. What is interesting is that it is called zinc phosphate but it much closer to manganese than zinc. Zinc would generally be much lighter like the middle leaf in the canter of the photo below.
FYI I tried some today on some very rough leaf springs I found.
The leaf spring on the left has a light coat of the zinc phosphate on the top half. The leaf in the middle is the graphite dry coating, and the spring on the right is the aftermarket leafs, not sure how old they are, but they were painted black at some point. No info on what springs they actually are.
Not sure why my photos are being turned sideways on the website