"Gunner Asch" wrote in message
...
On Thu, 31 Aug 2017 21:02:48 -0400, wrote:

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

wrote in message
...
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.

Well said!

Gunner, who does a lot of fiberglass work on boats

If I had a good recommendation for a specific epoxy auto body rust
repair product I would be trying it. Instead the paint dealers and
body shop estimators tell me -nothing- will last for very long in New
England. If so, I want something I can remove later to safely weld.

I have sandblasted coupons of Bondo Glass over etch primer, BG over
bare steel, and a rust converter outside drying right now, and a 5%
salt water solution to test them with. The letter stamps made a sharp
impression in the sheet metal against a backup of scrap aluminum.

I started cutting down the Bondo Glass at the point where I would
cheese-grator the plain stuff, and pulled the patch out of the
recessed hole. The break was in the resin, not from the unprimed
steel. The next batch hardened like concrete overnight. It does NOT
sand down as easily as regular Bondo. The 1/16-1/8" layer I left on
the mixing cardboard can be bent quite far before it cracks.
-jsw