i swear by the stuff, i've used a number of their products and have good results on a varierty of automotime surfaces.
but like anything else the better the prep the better the result, with POR-15 you can paint directly over a rusted surface and "emcapculate" the rust starving it from oxygen. the instructions are pretty straight foward, remove the flaky, loose material, wash the surface down with either their prep products marine clean or other (i swear by castrol supper clean) and paint. the product is a medium thickness and clings pretty well, it will self level wich removes the brush or roller marks. it can be sprayed but requires thining, 10 percent i belive with their thinner. POR-15 is not UV stable and therefore requires a protective topcote if it will be exposed to sunlight, this can be done easily with spray bombs after it has dried to the touch but within 12 hour if i remeber correctly.
i've coated the following
the bottom of my new cab, the inside flour is next
the wheel wells and back frame of my 78 rc
the new spring packs for the 78
power stearing pumps and other engine components
bumpers
the spring steel gas tank straps on the 78
the inside of my new replacement fenders for my 92
the list goes on
i haven't seen any cracks or peeling, you don't need to but i prefer to take most things down to bare metal