Por 15 will fade in areas exposed to sunlight. Its a great product to seal surface rust. I have used quite a bit of it in its proper application. Doesn't work as good on non rusted application.
I have great sucess with rustoleam
paint and primer but you really have to clean your work and then use the rusty red primer first. One good coat of rusty red primer and then 2 coats of
paint. Then maybe every 5 years a quick recoat on visible areas to keep it looking good. Never had it chip or peel in using the stuff in over 40 years. All with spray cans. If your using allot of it and brushing the paint don't buy it in larger cans than quarts if you live in environmental states. They sell an inferior version in the larger cans to meet regulations and it sucks.
If you want something better Eastwood has products for frame and chassis that work well and they have an internal frame coating with an application hose to spray inside the frame to seal and protect from further corrosion. It works well and does what it says it does. But you have slowly to get proper coverage.