I have never heard of the bicarb part of that treatment...
but anything which comes off in chunks or layers indicates an adhesion problem....not a substance problem...
If it were powdering, or rusting under the application I would suspect the covering...
I only sandblasted my last test... the bottom of a lawn mower deck.... have not put it back together yet.. BUT the fingernail test indicates the stuff is really hard...
And I know that there are decades of good reports concerning POR15 from the people who restore cars and other metal stuff who congregate around Hemmings Motor News...
Prep is everything on any paint... but may be exagerated concerning POR15... but they are very explicit in the directions which come with the cans...