Technical Outer to inner wheelarch joint...how best to bond it?

[fiat500]

I'm about to start on the laborious job of replacing the edge of an inner rear arch and the flared part of the outer arch.
Last time I did it I had no options other than to join the two flanged edges using a series of "puddle" welds to emulate the original spot-welds as closely as possible. Despite the application of plenty of protective primer and cavity wax, after ten years of use I'm going to have to replace almost exactly the same amount and areas of metal as previously.
It's a basic flaw in the design when considering corrosion prevention; many cars of this era are made in this way and I suppose ten years was a reasonable lifetime in the days of rust-buckets-a-plenty.

I've thought about adhesive bonding, but it's too technical, so my next thought is to carefully seam-weld the two edges. Although I would still have a hidden area with untreated steel or heat affected primer, at least it would be waterproof from the outside.

I'm wondering has anyone done this or got suggestions of an alternative approach?

 

[The Panda Nut]

Bonding materials that I have used would seem to do the job easily and cheaply. Give a good squirt and then carefully clamp the panels until set then apply seam filler agin and paint. Tiger seal is amazing stuff and there are profesional alternatives without gpoing mad on costs. I used tiger seal on a sill patch 3 years ago to seal it after welding and its absolutely solid still. I am no expert and am still totally satisfied with teh result.

 

[Goldnrust]

I presume from your commented about seam welding that you think the moisture is getting into the panels from the wheel arch side of the join, not working it's way down from above inside the car?

Beyond weld through primer followed by cavity waxes and other rust prevention I'm not sure theres anything that I can think of for solving the problem of moisture coming from the inside. Certainly seam welding would only make this worse as more of the paint would be burnt away in the difficult to rust proof areas.

Thinking of moisture coming from the outside, most modern cars still have arch lips formed in the same way, but to my amateur eyes what seem to be two key differences to help with external moisture entering the join.

The first is seam sealer that is seemingly applied between the panels before welding. With the small heat affected zone of a spot weld this still leaves seam sealer throughout the joint and makes it fully waterproof. I think trying to do that with puddle welds would just result in a lot of burning seam sealer, so possibly not a practical solution....

It might be possible to roll back the edge of the arch lip a little, so after puddle welding like usual you are left with a small gap between the edge of the panels all the way around, into which you could force seam sealer before pinching the two layers together? I didn't go that far on mine but I did run a smear of seam sealer over the join between the panels from inside the wheel arch to attempt to water proof that seam.

The other key thing modern cars have is plastic inner arch liners which wrap over this vulnerable seam and protect it from the majority of water/mud/stones/salt/etc. that comes off the road. Again, not sure how easy something like that would be to replicate? But there must be some slim rubber/plastic trim you could push over the seam and use a panel bond to hold it in place? The key would be finding something that isn't visible from the outside of the car, I'd rather have to do the wheel arches every 10 years that drive around with a big rubber strip showing on the arches haha.

 

[the hobbler]

I presume from your commented about seam welding that you think the moisture is getting into the panels from the wheel arch side of the join, not working it's way down from above inside the car?

Beyond weld through primer followed by cavity waxes and other rust prevention I'm not sure theres anything that I can think of for solving the problem of moisture coming from the inside. Certainly seam welding would only make this worse as more of the paint would be burnt away in the difficult to rust proof areas.

Thinking of moisture coming from the outside, most modern cars still have arch lips formed in the same way, but to my amateur eyes what seem to be two key differences to help with external moisture entering the join.

The first is seam sealer that is seemingly applied between the panels before welding. With the small heat affected zone of a spot weld this still leaves seam sealer throughout the joint and makes it fully waterproof. I think trying to do that with puddle welds would just result in a lot of burning seam sealer, so possibly not a practical solution....

It might be possible to roll back the edge of the arch lip a little, so after puddle welding like usual you are left with a small gap between the edge of the panels all the way around, into which you could force seam sealer before pinching the two layers together? I didn't go that far on mine but I did run a smear of seam sealer over the join between the panels from inside the wheel arch to attempt to water proof that seam.

The other key thing modern cars have is plastic inner arch liners which wrap over this vulnerable seam and protect it from the majority of water/mud/stones/salt/etc. that comes off the road. Again, not sure how easy something like that would be to replicate? But there must be some slim rubber/plastic trim you could push over the seam and use a panel bond to hold it in place? The key would be finding something that isn't visible from the outside of the car, I'd rather have to do the wheel arches every 10 years that drive around with a big rubber strip showing on the arches haha.

We have used a modern metal/metal adhesive in our workshop when doing some small repairs on classic cars---very happy with the result as no heat put into a (sometimes delicate) panel. Let's face it, MANY modern, VERY high performance cars now use the 'bonded chassis' technique that Lotus developed for (initially) the Elise----and I have NEVER heard of a bonded chassis 'coming apart at the seams' under normal use. I would look into the use of a modern adhesive in this situation.

 

[fiat500]

Thanks for all the advice. Having said that, it's all things I have already considered. I think the point made, that if it has to be redone at a future date, so be it, is the one I will have to bear in mind. By that time I'll be employing someone else to do it!
The moisture definitely comes from the outside at the top of the arch and then travels internally to affect the end of the sill and the triangular panel near the bumper. I used plenty of epoxy primer and PU sealant last time, and other than at the very edges the metal is still well protected.
I'm a more skilful and patient welder now, so maybe that will help. But
I was pleased and surprised to see that the corrosion had not come from my previous welding.

 

[Bounding Bambino]

I dont have the product number off the top of my head, but in my build thread i discuss the 3D panel bond i used for doing the fender seams. Im not infront of my computer at the moment too look it up. But its specifically designed for structural adhesive metal to metal bonding. If 3M is not avalable in the UK, i know Lord, SEM and a few other mfgs make similar product.

 

[fiat500]

I dont have the product number off the top of my head, but in my build thread i discuss the 3D panel bond i used for doing the fender seams. Im not infront of my computer at the moment too look it up. But its specifically designed for structural adhesive metal to metal bonding. If 3M is not avalable in the UK, i know Lord, SEM and a few other mfgs make similar product.

3M 08115....thanks. I'll look into that.

 

[fiat500]

3M 08115....thanks. I'll look into that.

The problem with this product and all the similar fixes, including "Tiger Seal" or PU sealant as it's generically called, is that it's not intended for structural panels. The 3M is very expensive and requires an applicator gun that costs about £500. As a monocoque and a fairly sketchy one at that, all panels on a Fiat 500 are structural to a degree. I doubt that use of these products at the wheelarch edge would cause any sort of catastrophic failure in normal use and in a collision it would crumple in any case, giving no protection to the occupants. Loads of 500s will be happily chugging around with compromised adhesion at the wheelarch edge.

But to keep the integrity of the car, unless something else turns up, I'm going to have to stick with welding.

 

[Goldnrust]

Having some some (admittedly limited) research myself into panel binding adhesives my lay persons take on it was that it clearly can work but to be effective the panels need to join in the right way that puts the right loads through the join to play to the adhesives strengths, and along with this the metal needs to be correctly prepared.

My ultimate conclusion was that for me, I will stick to welding. The panels are designed with flanges suitable for spot welding, so welding seems the best way to continue.

I’m sure the next generation of car enthusiasts will come along and know all about modern adhesives and how to get the best out of them, but for now I stick with what I know.

I guess just the same as the generation before me is much happier with a carb, but myself I grew up with fuel injection and so it doesn’t scare me in the same way.

 

[Bounding Bambino]

Keep in mind, just incase there is confusion. A proper panel bond adhesive epoxy is completely different then a sealant. One is designed too handle shear and stress loads. And the other is just that, a sealant to prevent moisture ingress, and does not provide or designed to provide mechanical fastening. the 2 are not comparable. Please dont use sealers as a replacement for a panel bond.

Panel bond adhesives are only ever a 2 part "epoxy" product, and will never come in a standard caulk tube type product. The chemical makeup, composition, and bond strength can only come from a base/catalyst type product. They will never be 1 part / air dry products.


Yes, you can spot weld, then seal which would give a level of protection; a seam sealer between the flanges then weld would work. Some sealers are designed for weld thru duty..

The 3M product does it all in one step, but yes there is a price associated with the application tools, and surface prep is critical for proper bond. They make it available in 2 forms (the 2 part manual epoxy cartridge), and the 2 part auto cartridge to use with the pneumatic applicator. the manual epoxy cartridge style is swappable between many other auto body supply cartridges. the major players all use the same style cartridge, and the applicator gun ranges from around 50-100$ usd. A bit of an investment none the less. But the benefits of using panel bonding for applications such as this over welding is multi fold. To each their own though.
I went down the route for a few reasons.. 1st, because it allowed a secure bond along the entire length of the arch and is a stronger bond then spot welding when applied properly. 2nd allowed the entire arch flange to seal preventing any moisture ingress into the seam which is a bonus. 3rd, ive dealt with panel bonding on a oem manufacturing level and am comfortable with its use, but application and prep is key though..

Panel bonding is not to be conventionally used in impact structure areas such as crumple zones, high load structures, and sub frame assemblies unless designed to be from new. Obviously this doesn't apply to our cars, it was never designed to have impact zones or such lol. But for some of the specialty cars that use panel bonding for safety critical areas such as the alfa romeo c4 for example. The seams are designed with optimal surface area to maximize bond strength when using a epoxy vs, mechanical fastening. One thing to note, when the term "structural" is used in oem, it relates to safety critical and suspension components. for example on a uni body car, all parts are technically considered structural. But to a different degree subframe vs rear quarter skin. The skin does provide structure strength, but to a lesser degree then the sub-frame mounts or frontal frame rails or impact pillars.


I would check out and see whats avalable for weld thru seam sealers. That would likely be the easiest route.

 

[Bounding Bambino]

I just looked up "tiger seal" as i had never heard of it. Looks to be a conventional sealer.

Im surprised that they mention in the description that it "Completely and permanently seals and bonds body panels, trim and seams ".. and then says "Permanently flexible – never gets brittle". Those 2 things contradict themselves. You never want a panel bond that is flexible.

hmmmm...

Marketing dept is getting a bit adventurous. And i see the confusion.

I wouldn't recommend it for a bonding application. Im sure its a great sealer. And honestly if you did use it on the 500, the cars are so light, that its unlikely to ever cause an issue. But caution is to be had when substituting it for a oem bond spec repair imo. Especially on a heavier car.

 

[Goldnrust]

Tiger seal is a PU sealant, fine for gluing trim or body kits on with, but nothing like the strength being discussed with welding or panel bonding.

It certainly sounds like you have the experience, with phrases like “ive dealt with panel bonding on a oem manufacturing level” that’s clearly beyond your average amateur like me, that’s why for now I’ll stick to welding.

Weld through seam sealer certainly sounds interesting and worth some investigation though! Too late for my build but there’s always the next project…

 

[Bounding Bambino]

There are alot of weld thru products that have entered the market. Honesty they still surprise me. To have a product that doesn't turn to a crisp when blasted with welding heat and still provide protection is interesting none the less.

not to toot my horn lol. I did some consolation work for honda and tier1 oems for a few of their retooling jobs for the assembly lines here in ontario. revolved around the frame assembly stations and the panel bond application robots. Alot of the more economy based vehicles are starting to see epoxy bond tech grow into newer assembly techniques. Its cheaper/faster to have a single robot spread on some schmoo, then 10 spot welding, and cure times can be very fast. Plus get added strength and less metal warp/heating.. Sometimes actually lighter due to design options stemming from it. Also can attach incompatible metals (aluminum to steel) and vise versa.

They save 50lbs by optimizing seam stresses/materials and such, then throw in another 70lbs of airbags, and collision sensors lol.