Nothing between the leaves? BMC products are certainly going to be different but on TR leaf springs there are typically rubber buttons between the leaves and of course... those perish over time resulting in the droopy, severe negative camber appearance of older cars. Teflon or Delrin buttons are installed to restore the original ride height. Again, probably no such application here but it's interesting that the use of leaf springs is so different.
I have a love/hate relationship with POR. You will only get good results if you follow their directions to the letter and really pay attention to the prep work. Sand blasting will create a tooth for the POR to adhere to. If you already have the leaves apart, by all means blast away and treat with the POR Marine Clean followed by heavy etching with their Metal Ready. You WILL need two coats. The first coat can leave pinholes for moisture to get through. Plan on painting being an all day job as you MUST let the first coat become sufficiently try (finger drag test) before applying the second coat. If you put the second coat on too early, trapped solvent in the first coat will blister through the second coat leaving tiny pin holes. When the second coat reaches the finger drag point (almost cured) apply a fog coat of the chassis black you want to use. Just a fog coat, no more. The next day, apply a regular coat of your chassis black.
As a footnote, POR only has about a 6 month shelf life, less once you open the container. DO NOT be tempted to buy a quart of the stuff unless you plan on doing a lot of painting in short order. A better investment will be to buy their mix/match 6-pack of small cans (half pints?). I will put down a color like grey or silver first and use black for the second coat so you know by the contrasting colors when you have applied a complete and uniform second coat.
Spraying POR also gives good results if you want to use it on surfaces that will be seen. You only need to thin it a tiny bit. I have never used it in a full sized gun. I have only used inexpensive single-action air brushes to apply POR. They work well and are easier to clean than a gun.
EDIT: And even though POR is tough, I would still consider at least putting grease on the contacting leaves of the assembled spring so the POR surfaces don't stick together.