POR-15 is great stuff, and apart from one fail, (and it happened 6 months after I sold the bike, so I don't know what happened to it after the new owner took delivery) and I've had it in my Suzuki GS1000 tank (which developed a leak after I painted it, so it was a real PITA prepping it for POR-15 with relatively fresh paint) for 5 years now, with no issues, and I did "Goldie" and my K1 at the same time 2 years ago, with no problems at all.
When POR-15 first became popular, it was God's gift to motorcycle restorers and the previous "fix", Kreme, became demonised. I find it interesting now that POR-15 has been around for awhile and a couple of new products have come along (Red Kote etc) that POR-15 is slowly becoming the new Kreme. (I've used Kreme, BTW, with excellent results, but I like the POR-15 color better)
The reason that the POR-15 didn't "stick" in your case mate, was that you didn't manage to remove all of the rust before you coated it. I saw that you had a really rusty tank to start with and you did a good job getting rid of most of it, but as Dave has previously mentioned, you need to get it to the point where there is no evidence of rust at all. The surface of the remaining metal might look like a moon scape, but that doesn't matter, the important thing is that it's all metal and there's no rust left.
It wouldn't matter what product you used in this case, they all would have failed. Cheers, Terry.