POR-15 is an excellent product, but not a magic potion.
First, it's a paint. Which means it needs to be applied over primer if used on clean, smooth sheet metal. This is true for any paint. If the surface is rusty, then all that needs to be done is to clean off the loose rust. The remaining corrosion pits leave something for the POR-15 to grip to (which is unique about it vs. normal paint topcoats.) If completely coated, it will seal the rusted area off from further corrosion. But without a primer or rough surface, it won't stick very well.
Second, to be effective, it must completely seal the corroded surfaces and any surface prone to future corrosion. If not, the rust will just submarine the POR-15 and rust out the surface from behind.
Third, as is well known, it shouldn't be used for surfaces exposed to sunlight (UV radiation.) If used in such an application, it has to be topcoated with something to keep UV rays away. Most undercarriage parts are fine.
Fourth, purchase the smallest quantity needed to do the job (or purchase it in small containers that can be opened as needed.) Once you open the container, if you spill any on the paint can lid sealing grooves or get it on the threads of a sealing cap, after a week it may be impossible to remove the cap or lid without destroying the container. The cured product is almost stronger than the metal container. Unopened it will keep for a long time. For small quantities, I use a large screw-type bolt to thread into the top of the lid, and unbolt it with a wrench to pour small amounts out. The screw action allows it to be unthreaded without destroying the lid.
It isn't a magic anti-rust potion. It is simply an extremely tough and durable coating that is fairly easy to apply, but unsuited to be a finish topcoat for anything where cosmetic appearance is important (i.e. visible body panels.)
I've used this to coat battery trays which were heavily corroded, and the coating was never damaged afterwards. I also use it on brake calipers (rough cast type, not forged billet) and it takes the abuse without failing. (though it's good to topcoat it in case of wheels that let a lot of light through.) On brake backing plates that were already heavily corroded, I scrubbed them and then topcoated with POR-15 - still in great shape after a couple years of daily driving.
If you spill some on a concrete floor, and it puddles up and dries leaving a lump - you can smash it with a hammer and it won't fracture or crack. It is useful if you're filling a gap with metal mesh, as an adhesive and binder (I often use it on composites instead of resins for small chipped/cracked/smashed areas.)
It would be good on a subframe if treated and applied like paint (part scuffed, primed, then topcoated with POR-15.) It would be as close to powdercoating as you can get if you don't have powdercoating equipment.
Dave
