does sunlight break it down if not top coated?
POR15 is a form of liquid urethane and yes direct sunlight will make it chalky and crumbly so it flakes off in a few months.
Additionally I've come to realize that if the metal is meticulously prepped and made shiny smooth...the POR-15 DOES NOT STICK. Yes it will APPEAR to adhere at first...but the meerest scratch or bump begins a lifting process ...any moisture brings rust which spreads quickly undernieth.
In my case the rusty metal around a windshield pinchweld was VERY carefully prepped with a power scotchbrite wheel...so it was smooth/shiny steel and 2 coats of POR15 brushed on.
Cured overnight.
Glued in windshield.
2 months later I installed a second windshield (unlucky bloke) and the POR15 was peeling off in sheets

It seems to stick best to rough rusty metal although a rough sandblasted finish should work well. I suspect smooth sanded primer would be a mistake although I haven't tried that.
(after years of using it off & on, I'm starting to experiment again as the peeling experience quite shocked me)
Please understand I'm Not Against POR15...but it's different than paint and follows a different set of rules.
"Sorta related info"...urethane in a much thicker form is used to glue windshields into cars since around 1980.
This stuff also deteriorates in sunlight.
That 2" black band you see surrounding the inner edge of your modern car's windshield, is called a "ceramic frit". It's main purpose is to block sunlight from compromising the urethane bond.