car rust repair

I have a persistent rust problem on the lower edges of rear fenders where the wheel well liner blocks access to the back side. The lower edge seam wasn't watertight and rust has perforated the outer and wheel well liner sheet metal below the factory sealer bead, which was applied from inside after spot-welding and didn't reach down to the joint. I dug out most of it so I could MIG weld patches without igniting it.
Several local paint suppliers have suggested 3M 08852 Cavity Wax Plus to coat the inside of my larger welded and smaller resin patches to prevent or slow the so far yearly rust lines along their edges. LPS-3 wasn't up to the task, or I didn't coat the out-of-sight area well enough with it. The Cavity Wax comes with long applicator tubes.
I'd like advice on if it really works, and how difficult it is to remove if for some reason it doesn't and I need to weld in a larger patch.
The clerk told me it's better than rubberized undercoating in the wheel well. The suggested alternative there is spray can truck bedliner, assuming it can be removed if necessary. tia -jsw

On Wed, 30 Aug 2017 11:20:00 -0400, "Jim Wilkins"
  Quoted Text Here  
If you don't get ALL of rhe rust out, it WILL come back - but a Wax-Oil reatment will drive out moisture and seal moisture out - doing a pretty good job of reducing rust. Bedliner just locks in any moisture amd lets it rot from behind leavinf you a plastic fender.

On Wed, 30 Aug 2017 21:27:09 -0400, snipped-for-privacy@snyder.on.ca wrote:
  Quoted Text Here  
How well does phosphoric prep/etch/rust-converter work in this application?
- I am a Transfinancial--A rich person born in a poor person's body. Please stop the hate by sending me money to resolve my money identity disorder. --anon

wrote:
  Quoted Text Here  
(Amazon.com product link shortened) "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.
When there is access to both sides I weld in steel patches, otherwise I sandblast all the rust off the exterior. In some cases my sandblasted repairs have lasted a decade, possibly because the back of the panel or bumper was still intact.
The problem here apears to be rust that broke through from the back, because the fender lip joint wasn't sealed well enough. I can barely see some of the damaged area from the inside, let alone sand it or scrub off all the factory caulk. I don't want to apply something flammable that I can't remove later to weld.
The dealer's body shop would replace the entire quarter panel, for more than the value of the car.
-jsw

"Jim Wilkins" wrote in message
(Amazon.com product link shortened) "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 :-).

On Thu, 31 Aug 2017 15:23:25 -0400, "Carl Ijames"
  Quoted Text Here  
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.

  Quoted Text Here  
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

On Thu, 31 Aug 2017 18:10:03 -0400, "Jim Wilkins"
  Quoted Text Here  
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.

  Quoted Text Here  
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

On Thu, 31 Aug 2017 22:00:00 -0400, "Jim Wilkins"
  Quoted Text Here  
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.

  Quoted Text Here  
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

On Thu, 31 Aug 2017 21:02:48 -0400, snipped-for-privacy@snyder.on.ca wrote:
  Quoted Text Here  
--snip--
  Quoted Text Here  
Good tip.
- I am a Transfinancial--A rich person born in a poor person's body. Please stop the hate by sending me money to resolve my money identity disorder. --anon

  Quoted Text Here  
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

On Sat, 2 Sep 2017 11:50:21 -0400, "Jim Wilkins"
  Quoted Text Here  
I gave you the recommendation. West systems epoxy and either chop mat ot woven glass mat well filled and pressed securely into place over the perforated metal, well cleaned on both sides if possible. Let the resin soak through and get onto the back of the body material.
When the subframe rail on my 1980 Corolla rusted through - along with part of the floor, I cut out the rail and replaced it with a factory repair part by welding it to the solid framework ahead, and to what was left of the floor. I then laid in a few layers of glass mat, sayurating it well, and then applied the blue "shop cloth" paper towels top and bottom,and then plastic film, and squeazed the patch to make sure the resin had completely folled the mat and all air was aqueased out. The shopcloth was fully saturated when finisged, and was left in place. That floor patch lasted for years in the southern Ontario rust belt. I used the same method on the 88 New Yotker floor and on the TranSport after having the subframe welded.
  Quoted Text Here  
In the past I have repaired rusted bodywork just using bondo, and it popped every time. Even over brazed patches the bondo eventually pops.
I've never had a fiberglass patch pop, although I suspect it WOULD pop if the panel was bent in an accident.
  Quoted Text Here  
After doing some investigation it looks like "bondo Glass" is listed as waterproof, even though it uses polyester resin and talc. If it is virtually impossible to "cheeze geater" and a royal bitch to sand, it may well do the same job as the epoxy and glass I used, if properly applied. No guaranteee, as I have not tried that prodect
  Quoted Text Here  

wrote:
  Quoted Text Here  
I agree there are others out there that may be less expensived - or mabee just "cheaper". West is a known entity - really can't go wrong with it, while "brand X" may or may not be quality stuff.
If you have experience with another product that you know works, use it. I'm just saying if you want to do the job, I know THIS works.
The composite aircraft guys generally use West around here.

On Thu, 31 Aug 2017 12:52:19 -0400, "Jim Wilkins"
  Quoted Text Here  
Por 15 is not UV stable. It has to be topcoated.
  Quoted Text Here  
You can do the same - for a lot less. Just takes a lot of time and work. There is NO OTHER WAY to totally eliminate that rust. It's like cancer. With cancer you can shrink the tumour with radiation or chemo - but it will likely come back. Or you can cut it out. If by cutting it out COMPLETELY, the cancer can be cured. If any cancer cells get away, it can come back. Exactly like rust

On 8/31/2017 5:20 PM, snipped-for-privacy@snyder.on.ca wrote: ...
  Quoted Text Here  
...
I'm late to the party, but I have some personal experience here. Back in '03 I painted a trailer frame with POR-15 and did not top coat it. (An erroneous page on their web site said that it didn't need to be top coated.)
The POR-15 that is on the part of the frame that is exposed is intact and there is no rusting there, or anywhere. It will likely fail sooner than if it was top coated, but I'm pretty satisfied with the 14 years that I've gotten.
Bob

wrote:
  Quoted Text Here  
I ended up brushing a similar Rust Converter into the gap between the fender and the wheel well edge, then sealing the openings in the highly curved area with SS heat treat foil stuck on with truck bed liner. Then I sprayed 3M Cavity Wax on the repair and the opposite side wheel well which didn't have the gap. Time will tell...
-jsw

On Sat, 30 Sep 2017 19:01:11 -0400, Bob Engelhardt
  Quoted Text Here  
Did you tell them of this, and did they offer a quantity of top coat (or top coat and por15) to make up for the confusion? That would be cheap advertising and extremely good customer relations to do so for anyone who purchased product in that time frame, despite no known losses by the consumers. It would lock in consumer loyalty as well as begin free advertising by said consumers to future consumers. Just curious. At the price they're getting for the product, they can afford it, too.

wrote:
  Quoted Text Here  
POR15 followed by Black-Kote