"Jim Wilkins" wrote in message news
https://www.amazon.com/Permatex-8177.../dp/B000HBNX38
"Not recommended for automotive exterior sheet metal repairs."

When POR15 first came out I called their tech help number and asked
about it for exterior rust spots. The girl said it wasn't recommended
for that use, but didn't know why, nor have I seen a reason more
recently.
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According to the can label and their website, POR-15 is not UV resistant so
it won't stand up to sunlight. It's ok on the underside of a car but not
the exterior. They have another product, POR-16 I think, that is more
expensive and rated for sunlight. A friend here in Maryland with occasional
winter road salt used a couple of quarts of POR-15 and some fiberglass cloth
to patch large rust holes in his Dad's Explorer, and we were amazed at how
well it worked structurally. Holes as large as 6"x6" in the wheel wells and
fenders. No peeling or cracking or any signs of further rust under the
POR-15. Four or five years later is still looked like the day he put it on,
but sadly his brother totaled the Explorer and stopped the test :-).

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Regards,
Carl Ijames