Why is POR-15 so frowned upon?

Thank you to all who have posted. After watching a blogger rave about POR-15 I had to dig in and see for myself. I have however posted a comment on the YT site, but I doubt it will be read. The person was reckless, no mask, no face protection no skin protection, he was treating this just like latex paint, I now understand it is far from that.
Also, no prepping, just slap it on rust in the undercarriage, and this is what really got me thinking. How!! just hard to believe that a product could work so easily.
Simply it does not work in this fashion.
Rust is an animal and it is alive, it is part of everything around us. It is either abated or it will live on and do what it has to do. Reclaim!!!
Once we trap rust it will do unbelievable damage. Do we want proof, look at the condo in Miami, this is what rebar does when rust expands and blows out concrete.
Take rebar, it is also sold with a green-coated layer, great until the layer is damaged and water gets in behind the coating. Then boom. For 100% rust prevention in concrete work, we need to use galvanized rebar.
As far as painting goes, maybe in areas where a clean finish is not needed, you spray paint to match. For showroom finish, no way period.
The comments I have read about cracking. Anyone here ever see a lousy driveway sealing job? There are sealers to chemically join with asphalt and then here are those that when dry scale and crack and the driveway looks like crap. And water will get through those cracks, I gather this is what POR-15 will do.
Conclusion: Having read all of the posts I will not donate $50 a QT to hype.
 
Another thing I've been curious about is "sealing" metal. Rust is oxidation, which needs oxygen to happen. Let's say you have a rusty piece of sheet metal, just a square chunk for ease of conversation, and coat each side completely. I don't care if you use POR-15, epoxy primer, whatever, but each side is coated. Now, this is where I may be off, since the rust is sealed from oxygen, how is it able to continue rusting?

Is it because none of the coatings can seal over rust? I understand polyester primers, body filler, etc etc all "breathe" which would allow the oxygen to reach the metal so it could further oxidize, but don't coatings like epoxy, POR-15, etc etc all seal the metal from oxygen? Please, please don't read this as me justifying painting over any kind of rust. I haven't and don't plan on doing this. My use of POR-15 was always under the assumption that it was very chip proof and sealed well, on pieces that didn't see sunlight and were previously sandblased or clean bare steel. I'm purely curious as to how rust can continue to form when it is sealed off from oxygen.
Oxygen is not the cause of rust. This from my Navy training, the metals we use are not pure. There is an electrical potential among all the different elements in the alloys we use. Water with impurities will conduct electricity and is therefore the agent seen to "cause" rust or oxidation of the iron in all the iron alloys in common use. The proper way to think of corrosion is to think of millions of tiny galvanic cells of iron and carbon - charged batteries - that cause the iron atoms to oxidize when they conduct electricity. If you can protect the alloy from contact with a conductor such as water loaded with conducting ions (pure water will not conduct electricity), you prevent "rust" same as preventing contact with oxygen. If you put the metal inside a sealed dry container, it will not corrode. However, humidity in the air is also water that will condense onto metal whenever that metal is colder than the humid air. There was a statue or some such discovered in India that was thousands of years old and not corroded because it was pure iron. How those old smelters got it that pure is a mystery. We have to work hard to purify metals and other elements these days.
 
Oxygen is not the cause of rust. This from my Navy training, the metals we use are not pure. There is an electrical potential among all the different elements in the alloys we use. Water with impurities will conduct electricity and is therefore the agent seen to "cause" rust or oxidation of the iron in all the iron alloys in common use. The proper way to think of corrosion is to think of millions of tiny galvanic cells of iron and carbon - charged batteries - that cause the iron atoms to oxidize when they conduct electricity. If you can protect the alloy from contact with a conductor such as water loaded with conducting ions (pure water will not conduct electricity), you prevent "rust" same as preventing contact with oxygen. If you put the metal inside a sealed dry container, it will not corrode. However, humidity in the air is also water that will condense onto metal whenever that metal is colder than the humid air. There was a statue or some such discovered in India that was thousands of years old and not corroded because it was pure iron. How those old smelters got it that pure is a mystery. We have to work hard to purify metals and other elements these days.

Interesting, learned something new today.
 
Hello everyone, complete noobie here! I stumbled across this thread while trying to figure out what to do with the underside of my car. I have previously used POR15 many times, and for a long time I thought it was the bees knees. I'm not a pro in paint or body work, just an amateur who wants to make his cars last forever, and is willing to put in the work to make it happen. I thought that if I followed all the POR15 directions down to a T I would be rewarded with awesome long term protection against rust. But I saw and heard things over time that made me question what POR15 could actually do, and now I'm at a point where I kind of lost and need some help.

I'm in the middle of doing a mini-restoration of my 1996 Infiniti I30 that I've owned since 2003. I cut out all of the rusted metal and taught myself how to MIG weld to patch it back up. But now I need to figure out what to do with those areas under the car that I've repaired, and also the areas around them. I scuffed up the new metal and wiped it down thoroughly with PrepAll, and then sprayed everything with some 2K Epoxy primer I got from my local paint shop. I then sprayed some high build primer on top of that to smooth things out, and that's where I currently stand.

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I know I need to get some paint on top of the primer, but I don't know how to approach all the surrounding metal. There is surface rust here and there, but nothing some sandpaper won't clean up. These are the areas where I was planning on using POR15, but it sounds like that's not a great idea. I guess my question to you is how should I approach those areas? Is the answer always "take it down to bare metal and then epoxy primer"? I feel like removing what's left of the factory coating is counter productive, since it's still in good shape in a lot of areas. How should I approach these mixed areas where I have fresh metal, old metal with good factory coating, and old metal with some light surface rust?
 
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