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On Thu, 31 Aug 2017 22:00:00 -0400, "Jim Wilkins"
wrote:

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On Thu, 31 Aug 2017 18:10:03 -0400, "Jim Wilkins"
wrote:

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On Thu, 31 Aug 2017 15:23:25 -0400, "Carl Ijames"
wrote:

"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.
============================================ =================

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
:-).
Por 15 has (or at least HAD)"BlackCote" which can be applied
over
the regular POR 15 to make it UV stable. Cannot be used alone.

The advice I got from several paint shops was that Bondo Glass or
equivalent was the right thing to fill sandblasted pits and small
holes, with or without a thin first coat of etching primer. Then
the
Cavity Wax would seal any perforations from the back side.

I cut some test coupons from new and rusted sheet metal to check
the
long term durability of several coatings, but I have to redo the
car
fairly soon with the best-sounding options before New England
weather
cools too much. On August 21 I did a quick exploratory repair
using
easily removed high-build primer as the filler and it's already
showing rust lines.

The sandblasted pits and welded patches I did on my truck 10-15
years
ago are still rust-free.

-jsw

If you are patching with a fiberglass or plasric filler make sure
it
uses an epoxy resin, not polyester. An epoxy/glass filler seals
the
patch. Polyester filler is porous and does not seal out water. You
can
use bondo to profile after the hole is patched.

I did ask about that, and was told Bondo Glass is "different", and
somehow better, though it still needs rust converter or cavity wax
on
the back to seal out water. It seems to be short fiberglass strands
in
mainly dark green resin with little or no talc as in regular
bondo-type putty.

Two years ago I used JB Weld epoxy to fill the small holes. By
spring
they were outlined in rust.

-jsw

It's still polyester Use marine or aircraft product - west system
epoxy and chop or woven glass mat to fill the hole. Cut out as much
rusted steel as possible and "bump it back" to make room fot the
filler. Grind the patch to flush or just below, then top with bondo
and sand to finish.Epoxy/glass is a bit of a buffer to sand out
smoothly.

I follow the Darwinian theory that the specialty auto parts suppliers
know and sell what's proven to work, if I can learn how to use it
correctly, and I haven't found an epoxy-fiberglass product in any of
them, although I've heard and read that it's better than polyester
since I started patching rust in the 1970's. JB Weld failed as a rust
pit filler for me.

I'm now experimenting with sandblasting and patching 1/2" punched and
depressed holes in a rusty piece of water heater shell in various
ways. Yesterday I applied Bondo Glass over one coat of etching primer,
today it will go directly on the sandblasted steel. Bumping back the
edges without warping the panel is much easier over the hardy hole on
the anvil.

-jsw