Repairing roof pinholes with 3M 8115 Panel Bond

Dean Jenkins

Promoted Users
I had 4 or 5 rust pinholes in the el Camino roof. I was stressing a bit about cutting the metal out and trying to butt weld in patches.
As has been discussed many times in this forum, doing flange type lap/fillet welds will result in ghost lines due to different metal thicknesses. And this car will be black - the worst :cool:

I was really worried that attempting butt welds would result in blow throughs on partially compromised metal. And of course, warping out in the middle of a big panel.

Luckily, I've made friends with a local restoration shop owner who has offered to coach/mentor me as I learn.

I called him and asked about this. He offered a simple solution. He said, "you are right, that is a VERY advanced welding challenge - I wouldn't attempt it. Use 3M Panel Bond. Grind the underside to bare metal and then spread the panel bond like body filler and let it ooze up through the pinholes, then grind any excess on the top, down flat after it has cured." He even loaned me his special 3M 2-part caulking gun to do it (~$60 to buy one.)

I had never heard of this material. After reading about it, I was amazed! Many of you already know, but they use this to build new cars, in liu of welding. Wow!

So I did it. And, we decided to use it on the factory seam between the quarter panel sail and the roof panel. The factory joint was filled with solder. We melted the solder out to check for rust. The indent is about 1/4", which is deeper than I wanted to fill with body filler.
So, filled about half of the depth with the 3M 8115 and then can skin the remaining with body filler. Nice!

Here are a few pics:
Hard to see the holes. I turned the shop lights off and used a mega halogen work light in the cab to make sure I found them all.

Roof pin holes.jpg


Skim of 3M 8115 on the inside of the roof panel

Roof skim.jpg


Quarter panel sail to roof joint, bare

Sail to roof joint.jpg


Quarter panel sail to roof - skimmed with 3M 8115

Sail skim.jpg


I think this is going to work!
 
I have used lots of 8115, but I have never had any luck using it on a seam that showed. Maybe I did something wrong, for your sake.
Thanks. It won't be showing, will be skimmed over with body filler. Just wanted a harder stronger material under the filler given the depth of the seam. Will post results when we get there.
 
but they use this to build new cars, in liu of welding.
Be very wary using it this way. Although some market it as a substitute for welding, it is not. It is actually used as an addition to welding. If you read the TDS you'll find that welding is recommended/required in addition to the panel bond. And never on anything structural. The only place I have ever seen OEM's using it with any regularity is bonding door skins to the door frame.

Just a friendly note, I would not prime over that inner roof structure without getting that metal cleaner.

Be prepared for that seam to show when it gets warm. (out in the sun) Due to the nature of that joint it doesn't matter what you use it will show to some extent. Metal will expand when warm more than the filler material will. Only way to stop it is to change the joint. Cut out the joint and butt weld a patch. Not very practical and almost no one does it.

Lead done right is probably the best method to fill the seam but it will show as well. And then there is the issue of folks not knowing what to use to sand a leaded area and what not to use. Using aluminum oxide paper to sand lead will result in something called intergranular corrosion all over the leaded area. That condition is often mistaken for the seam showing through. Short of replacing/ reworking the seam, lead is the best choice, just as the OEM's did.

Body files and silicon carbide paper only on lead.:)
 
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3m 8115 is a panel bonding adhesive, NOT a filler. I use it to bond fiberglass panels on my vettes. I used it as you are doing ONCE. never again. It will shrink over time and your nice perfect paint job will be sunk in where the 8115 is. I would use a something like vette panel adhesive or some other high strength filler. Good luck.
 
You will probably get away with what you are doing filling a few pinholes on the roof imho. The sail panel is risky as said above imho. It doesn’t add a lot of strength in that application. I like to use SEM Carbofil there. It’s a carbon fiber reinforced filler.

Don
 
You will probably get away with what you are doing filling a few pinholes on the roof imho. The sail panel is risky as said above imho. It doesn’t add a lot of strength in that application. I like to use SEM Carbofil there. It’s a carbon fiber reinforced filler.

Don
Don, thanks. Based on yours and some of the other comments, I'm having second thoughts about the sail panel. The joint is welded from the factory so I think it is strong. I'm worried about shrinkage of the 3M 8115 since it was not meant as a filler.
I will probably grind it out and use All Metal Body Filler for most of the seam and then a final thin coat of regular filler for smoothing.

As Chris points out, I will probably end up with ghost lines when it is in the sun, but that will be the case with any material and would have been the case from the factory, given the differences in thickness of the joint vs. surrounding metal.
 
Some guys like Allmetal but I hate that stuff. It’s just regular filler with some aluminum powder tossed in for marketing purposes imho....
 
It's obvious you arent using it for strength or to hold the seam together just filling in the pin holes which in my opinion is just fine as the those bonding adhesives are epoxy based. Just sand down the excess and move on with your normal fill work. I'd personally start with a quality duraglass filler to to fill the main void. since you obviously cant lead anything in with that epoxy in place. If you fill that entire void with a standard filler you will run the risk of the it ghosting back.
 
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Be very wary using it this way. Although some market it as a substitute for welding, it is not. It is actually used as an addition to welding. If you read the TDS you'll find that welding is recommended/required in addition to the panel bond. And never on anything structural. The only place I have ever seen OEM's using it with any regularity is bonding door skins to the door frame.

Just a friendly note, I would not prime over that inner roof structure without getting that metal cleaner.

Be prepared for that seam to show when it gets warm. (out in the sun) Due to the nature of that joint it doesn't matter what you use it will show to some extent. Metal will expand when warm more than the filler material will. Only way to stop it is to change the joint. Cut out the joint and butt weld a patch. Not very practical and almost no one does it.

Lead done right is probably the best method to fill the seam but it will show as well. And then there is the issue of folks not knowing what to use to sand a leaded area and what not to use. Using aluminum oxide paper to sand lead will result in something called intergranular corrosion all over the leaded area. That condition is often mistaken for the seam showing through. Short of replacing/ reworking the seam, lead is the best choice, just as the OEM's did.

Body files and silicon carbide paper only on lead.:)
Chris, thanks good info. Was not attempting to use it structurally (the joint is already welded) but as a filler. Seems that that is not the intended use and it may shrink.
No way I'm going to attempt lead or butt weld. After some of the discussion here, I think I'm going to grind out the 3M and go with All Metal Body Filler, topped with thin coat of normal filler.
Yes, the whole car has to be prepped with 80 grit/grinder before epoxy primer. Going to finish all metal work first, then prep and epoxy.
 
I believe usc makes the all metal filler. Try to find the stuff from evercoat. I believe its metal 2 metal. I have not used the usc stuff but know people that have and they had ghosting issues with that. I have used a bunch of the metal 2 metal over the years and once on the sail panel where you are using it and i had zero issues. I dont think i just filled that in though if i remember right. I believe i cut it out and welded a piece in there. I always used metal2metal over all my welded areas to do the bulk of the filling or to smooth things out then went to regular filler.
 
I would have tig welded those tiny holes using heat dam and used Adtech P17 on that seam ...after sandblasting and epoxy! It's still rusty in the pic, and you're putting a filler over bare rusty metal. :oops:
 
Not everyone can TIG weld. Sometimes folks need to do the best they can do with the tool and skill set they have. No way you could find a shop willing to do that minor repair. A lot depends on the timeframe. Five years, ten years, 20 years. Smithsonian museum piece, show car, driver. What is the budget. First amateur resto or professional build. I’ve seen a heck of a lot worse come out of some “pro” shops....

Over on the Camaro forum there is an smug restoration shop owner who likes to dump on the work posted by hobbyists. Yet he says it’s ok to repair pinholes with panel adhesive. Go figure, I was surprised to say the least.

Don
 
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How I have done those in the past, clean the rust pinhole with a small drill bit (1/8"), then MIG weld it. Position the gun so the wire is in contact with the hole and just super quickly on-off with the trigger. On a larger one still super quick on-off with the trigger probably 2 or 3 times. Just fill it that way. Do it right very minimal to no distortion and after you grind it flush give it a few taps with a hammer and dolly and any distortion can be filled with a couple of coats of build primer or glaze.
And if you can position a copper backing pad underneath it is even easier to do it that way.
Not saying it's the best way but it's an option that is achievable for hobbyists.
 
I would have tig welded those tiny holes using heat dam and used Adtech P17 on that seam ...after sandblasting and epoxy! It's still rusty in the pic, and you're putting a filler over bare rusty metal. :oops:
Not to be argumentative . . . I REALLY appreciate this forum and all the really good information I got on this.

The owner of the most respected custom body and paint shop in the region is a friend, he came over and put his eyes and hands on the panel and said "I would not attempt to weld that." And suggested the 3M 8115 as a solution. Good enough for me.

And, the area around the pinholes was ground to shiny metal - per the 3M 8115 TDS. Yeah some lapped out at the edges to un-prepped area, but where it matters, it was done right.
 
These VIM magnetic plug weld tools come in handy when welding up holes with a MIG. I use them when welding up holes in firewalls. They even work well to hold small plugs in place for welding.

Don
 
A little 8115 on the inside won't hurt anything. On the outside, it will show. I'm assuming the rust pits are slightly lower than the surrounding metal, so after epoxy they can be fixed with some conventional filler.
 
Not everyone can TIG weld. Sometimes folks need to do the best they can do with the tool and skill set they have. No way you could find a shop willing to do that minor repair. A lot depends on the timeframe. Five years, ten years, 20 years. Smithsonian museum piece, show car, driver. What is the budget. First amateur resto or professional build. I’ve seen a heck of a lot worse come out of some “pro” shops....

Over on the Camaro forum there is an smug restoration shop owner who likes to dump on the work posted by hobbyists. Yet he says it’s ok to repair pinholes with panel adhesive. Go figure, I was surprised to say the least.

Don
This is a very valid post. I strive for perfection but like texasking said previously....Jesus shop is down the street...sometimes u have to ask when is good , good enuff? Well that conversation can go on until Jesus is returns....with so many products available there are many ways to skin a cat.
 
Going out on a limb on this subject but has anyone heard of this product?
The video is pretty crude but as most product endorsements go it's the cat's meow.

 
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