Rustoleum vs. POR 15 vs. others

I am not a Pro.
From what I have scene por 15 is seems to be a thicker material.
It held up real well on my buddy's 70 split bumper camaro.

they say Por-15 does not do well by itself in direct sun light
I recommend an epoxy based sealer in your primer and/or final coat.

I used Ospho and then 7 coats of Rustoleum rattle can on a trunk lid -
-to no avail, it still rusted thru in a few months...
...of living on the coast of the SALTy ocean.

Years ago, after the initial purchase of "Made in Mexico" leaf springs made out of regular production steel, I sprayed 3 coats of primer and 7 coats of a rattle can labeled "flexible paint" from the local advance auto department store -10 years of neglect and it has held up really well here- some rust in a few spots but overall pretty damn good.

(notice phosphate/Ospho Petroleum/Rustoleum in the names)

I believe that older acrylic paints would not bond to the oil based Rustoleum paints but am not sure how well the acrylic enamel blend paints would do-it has been suggested by bodyshop owners that know that they talking to a low budget guy that modern acrylic enamel paints should do just fine, I have not substantiated that yet-will let you know sometime next summer after deployment.

When I worked at the explosive handling wharf ( a submarine hanger on a pier),
stainless steel, non ferrous aluminum alloys, etc. all corroded under the sea's alkaline influence-so things tend to rust quickly here - I just lost the battle with mother nature with my 78 D300 which has gone the way of razor blades (RIP 77-2011)

I am still fighting this battle with 6 other Mopars, and there will be more casualties.
Those alloys on the pier never touched the brack water from the river; salt is in the moisture in the air here.

I am going to try the marine rustoleum (which has a marine rustoleum primer-available at my Lowes store) on my daily driver soon.

I believe an etching primer is a better start for bare metal, but i would not necessarily remove all of the paint from a non-rusted surface (body shops do remove all paint as a general practice) -I don't because I find that the Ospho penetrates the paint-if there is any rust, the Ospho reacts to it thru the original paint.

wiki: ...Metal primers might contain additional materials to protect against corrosion, such as sacrificial zinc...A steel part can be rusty, for example. Of course, the best solution is to thoroughly clean the metal, but when this is not a viable option, special kinds of primers can be used that chemically convert rust to the solid metal salts. And even though such surface is still lacking in comparison to the shiny clean metal, it is yet much better than weak, porous rust. Painting and gluing aluminum is especially important in the aircraft industry, which uses toxic zinc chromate primers and chromating to add the necessary adhesion properties.

A galvanic anode is the main component of a galvanic cathodic protection (CP) system used to protect buried or submerged metal structures from corrosion.

They are made from a metal alloy with a more "active" voltage (more negative electrochemical potential) than the metal of the structure. The difference in potential between the two metals means that the galvanic anode corrodes, so that the anode material is consumed in preference to the structure.

The loss (or sacrifice) of the anode material gives rise to the alternative name of sacrificial anode. -We use blocks of zinc on our high-yield 80 steel submarines to combat the hot sea's we sail in. When we return to port, the zinc is completely coated with salt. The Trident subs here in the southeast, rust so much more in the southern Atlantic, that our budget to maintain them is nearly twice as much as those in the northwest Pacific for the same ship deploying under pressurized salt water for the same amount of time. Lead based epoxy paints protect our subs.