I have used POR-15 for over 25 yrs and some of those original repairs are still good. More recently I have used a similar product called DOM-16. These are single part polyurethane paints that use moisture from atmosphere for curing. The Eastwood rust encapsulator sounds like something similar. There are others too like the Miracle Paint Ken at MercSource pushes.

One problem with this type of paint when used for external finished surfaces, is that it cures to a very hard finish. It is very difficult to fair into surrounding areas for painting. I have used POR for repairs like yours, but only applied a relatively thin coating to seal the surface after first grinding out all the rust. If a hole developed, I would use the POR to apply a patch on the other side of the metal using a small piece of fibreglass cloth or mesh. After that I have sanded POR to roughen and used either 2-part Bondo or a 2-part epoxy fairing compound that can be sanded and faired into surrounding areas. Then prime and finish paint. Not easy for an amateur like me, so usually repair can still be seen,but at least the rust has gone!

When we first used POR-15, they didn't even sell all those pre-treatments that they now sell. So long as you get a good oil free surface with enough roughness for paint to grip onto, you can paint on the bare metal (just don't get metal too smooth)

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5