I have seen POR 15 crack and peel on numerous occasions. It is not designed to go over non porous surfaces. If using POR15 use it on rusty scaly areas as designed and then  use a proper primer and paint on the other surfaces and over the top of the Por15 , as I understand it is not UV tolerant as a top coating. 

                           BTDT 

                                 Stacy

Sent from my iPhone

> On Oct 13, 2015, at 6:46 PM, Nick Feickert <thesuperflydisco@GMAIL.COM> wrote:
> 
> Thank you for your concern regarding the use of POR-15. Yes, it's designed
> to be applied to a rusty surface, however, it's also rust resistant,
> corrosion resistant, and doesn't chip or crack. Which makes applying it to
> a propane tank that rides underneath a vehicle very practical. The amount
> of abuse the tank goes through under there requires a restoration be done
> with the thought that this kind of abuse will contunue. Why not give your
> LP tank some love and protection like a couple coats of POR-15. Just
> because I knocked most the rust off doesn't mean I'm doing the tank any
> harm by give it a nice suit of armor!!
> 
> Other notes:I plan to get the interior of the tank scoped before I
> purchased any new hardware for it. Just to make sure it all looks good in
> there.