The can says it must be topcoated for prolonged exposure to sunlight.

Has anyone forgotten to topcoat it - if you did, what happened?

And any idea what prolonged means in this context? Like for example exposed chassis rails in a Westfield cockpit that is garaged most of the time but sits in paddocks over the summer for a dozen or so Saturdays or Sundays?

My Sylva J15 (roll over bar) has been coated with it for a year. It was the two pack version. Great stuff better than powder coating and much easier to touch in due to engineering damage etc.

I believe it turns a dark green colour over time ?

When i've used it on Westfield chassis rails inside the cockpit i've always overcoated it with Hammerite satin black which gives them a nice 'powdercoat' type finish.

Unless its outside most of the time i'd of thought the effects would have been negligible ?

Also habe a look at KBS rustseal. Very similar to POR15 but can be used on clean metal. I am using this after preping with POR marine clean and metal prep (kbs version not in stock). This has a similar warning on colour fastness but Ok for underbody and chassis.

I used it on a Tiger chassis, wishbones etc about 15 years ago. I did not overcoat it and after about 18 months it started to take on a yellow/green tinge.
There didn't seem to be any breakdown of the paint itself, it was simply the visual effect. After another 18 months or so it didn't seem to get any worse but I then sold the car.