years ago I used a product called "Body Schultz, its still available, you use an undercoating gun, there fore get adequate thickness, that you don't get with a spray can. ya might check it out, its used by pro shops.
like he said undercoated cars by the factory, the bottom of the car lasts lot l;onger, a fact, ..... if moisture getting on the top side of floorpans they will of course, rust from that side! yes undercoating has weight, racers burn it off just like the resto guys do so he can have a SHINY bottom on his car!????? ( then he won't drive it!?????)>>
I have had good luck with POR 15, don't like the price but the I don't like the price of epoxy primer either, but the good stuff ain't cheap!??
bottom line is anything we can do to remove, neutralize rust ( with acid or a inhibitor), prime with material that adhers, seals out moisture. gives foundation for a sealer and or apint, then we take care of the car and keep it out of rain and snow, should last many years....
we have to realize some primers , like red oxide,,, etc. don't keep out moisture. but epoxy primer is used as a sealer buy many pros. but again not cheap.... I have used oil based equipment paint, rustoleum I assume would be an example, x o rust enamel. I like Valspar oil based enamel. I've used these on inside floor pans, that I completely rid of rust, used acid or inhibitor,for that.... then enamel, now I started using sound deadner on that, works great for me... pros would clean the fllor pan, spray epoxy or self etch primer, paint over that....