I've been doing research on the POR-type products, and one way to really get to the heart of the matter, is look at the MSDS sheets for each one.
One will find that products like POR-15, Rust Bullet, Chassis Saver, Master Series, etc. are all basically the same formulation with minor changes- the main reason that Rust Bullet has a better rep lately is, they only offer it in silver- with a high aluminum content. Any aluminum paint is by definition a powerful paint-over-rust paint, ask any midwest farmer who repaints his fencerow posts. If they are metal, simply painting them with aluminum paint stops any further rust- because the aluminum shavings in the paint, seal off the parent metal- and aluminum is a powerful anti-oxidant, it doesn't rust.
Reading around the automotive forums, the general consensus is, the best moisture curing urethanes from each make, are the silver colors- intuition tells me, it's the aluminum in the silver paint that makes it better. It very well can't oxidize or chalk, if aluminum is the pigment to begin with. The urethane feature merely gives it the hardness.
This led me to Zero Rust, which is a different product, their website says it's a "phenolic modified alkyd coating". Everyone is raving about how much easier it is to spray and use.
Well a lightbulb went off, because I studied different types of paint formulations before- "alkyd" is synonymous with "synthetic enamel"- a very old paint formulation, one of the first. It's been around since the 1930's.
Zero Rust is basically a paint like XO Rust or Rustoleum, nothing more. Rustoleum is a "phenolic modified alkyd" too. There's a reason why Zero Rust sprays on so easy, but will chalk up and has no UV protection.
I downloaded the MSDS for Zero Rust, from their website. I compared it to MSDS for Rustoleum.
It's the same stuff.
Zero Rust is nothing more than a Rustoleum type paint. Yes, it is quite good if used intelligently, just like Rustoleum was/is. I blasted the entire underside and frame of my GTO nearly 25 years ago, and painted it with Rustoleum- it's still the same as I painted it, today- because the car was rarely driven, and now garaged, and the UV could not get at the underside. I painted the entire BODY of that car as well- the sun chalked it up, then it began to rust- broken down from UV and rain, from sitting outside. Since then I blasted it clean again, and self etched it, and garaged for the time being.
Back to back tests with Rust Bullet against another "phenolic modified alkyd coating" showed the alkyd was actually inferior. The 2000 hour salt test result for Zero Rust can be deceiving- real sea water and water with table salt added, are 2 different things.
2000 hours on salt tests may seem like a lot for Zero Rust, but Master Series has survived the salt water test in excess of 5000 hours. But Master Series is the same darn thing as Rust Bullet anyway. One thing is certain, POR-15 seems to be somewhat inferior to the newer Moisture Cured Urethanes.
My gut feeling is, the industry as a whole has gotten carried away a bit with the POR type coatings- there was money to be made on car restorations, where people were lazy or not well equipped or on tight budgets, and could not afford to dip/sandblast. So it went full circle that now ease of use has displaced the hardness and finicky nature of the POR type urethane coatings, so now we're back to square one using a Rustoleum type product again- but just calling it a different name- Zero Rust.
The maker of Rust Bullet attested to this, saying there are now 319 different types of POR type products, but they all use the same 6 ingredients. When one is tested and shown to be inferior, before the results can come out, the inferior product is repackaged, put back on the market with a new name, and resurfaces.
My friends, Zero Rust is nothing more than Rustoleum. It's an oil based modified alkyd enamel, using butyl acetate, naptha, methyl propyl ketone, xylene, with additional dispersants and driers.
Just like urethanes, how new it's not.
I also have a gut feeling, that merely mixing up a batch of black Imron with hardener, and painting it on rust in thick coats, would work nearly as well as any POR type coating. It's the same stuff basically. Any urethane is attracted by moisture, and adheres/hardens to it- which is why it attacks the painter's eyes and lung linings so viciously, if given a chance.
If you do a search for "phenolic modified alkyd coating" on Yahoo, you'll find that a bunch of paint mfrs. make such a paint, and it's basically a marine paint. Nothing new.