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Protecting inside of frame rails


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I’m at the stage where I’m about to install bad dog rails and sfc’s on my 280z for stiffening purposes (original rails are fine). I’ve read every thread on the subject I can find but have found some conflicting info. I know the replacement rails can be installed over the existing ones, my question is does this create a potential trap for corrosion? Also when leaving the originals in place does it make more sense to drill drain holes in the new rails or try and make the whole thing airtight with seam sealer after welding?

 

My tentative plan:

1. Blow out and shoot internal frame sealer in stock rails. 

2. Clean the outside and mask off the weld areas and spray epoxy primer. Do the same for the interior of the new rails. 

3. Fit new rails, weld, epoxy the outside, then seam seal. 

 

Any input appreciated, thanks. 

 

*Mods - I wasn’t sure if this should go in fabrication or paint and body, feel free to move. 

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I think you're right about creating a space for corrosion, but if you look, the whole chassis is built by layering sheet metal and spot welding it together. My 70 is pretty rust free... except for all of those areas. I just cut the pass seat mount out. Rust between the seat mount and the floor. No rust anywhere else on the floor, as I fixed it years ago. That's the story for the whole car. 

My take is that trying to get the rails perfectly sealed is a fool's errand. For frame rails and hollow cavities, I think the best thing to do is shoot oil inside of them after welding. This is what Krown rust prevention in Canada does: https://www.krown.com/en/

I suppose you could spray oil everywhere there are overlapping sheet metal panels too, but there are a lot of them inside the cabin, and you'd have to dig out the seam sealer to get access. It would be messy. 

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5 hours ago, JMortensen said:

I think you're right about creating a space for corrosion, but if you look, the whole chassis is built by layering sheet metal and spot welding it together. My 70 is pretty rust free... except for all of those areas. I just cut the pass seat mount out. Rust between the seat mount and the floor. No rust anywhere else on the floor, as I fixed it years ago. That's the story for the whole car. 

My take is that trying to get the rails perfectly sealed is a fool's errand. For frame rails and hollow cavities, I think the best thing to do is shoot oil inside of them after welding. This is what Krown rust prevention in Canada does: https://www.krown.com/en/

I suppose you could spray oil everywhere there are overlapping sheet metal panels too, but there are a lot of them inside the cabin, and you'd have to dig out the seam sealer to get access. It would be messy. 

 

Thanks for your input and good points. I may be overthinking it. I’ve had similar experiences with minor rust between welded panels and mine is pretty rust-free otherwise. You drilled drainage holes in your rails, right?

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5 hours ago, jhm said:

This is a pretty good product for rust protection anywhere internal....comes with a 2' hose to get at all the nooks and crannies:  https://www.eastwood.com/internal-frame-coating-aerosol-black-14oz.html.

I do have some of that I was planning on using, thanks. 

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25 minutes ago, artishard116 said:

 

Thanks for your input and good points. I may be overthinking it. I’ve had similar experiences with minor rust between welded panels and mine is pretty rust-free otherwise. You drilled drainage holes in your rails, right?

I did. Haven't yet sprayed oil in there, but that's the plan. I am going to use Corrosion X when I eventually do it.

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  • 4 weeks later...

What I did/will do was this:

 

I drilled drain holes in the low spots of the Bad Dog rails, directly below where the OEM rails had the drain holes.  Then I sprayed the inside of the Bad Dog rails with epoxy prior to fitting them.  That should leave only the weld areas to have bare metal.

 

This might be a bit anal, but I am going to use a small brush with Rust Bullet or a similar product to get the cracks and crevices from the outside around the welds, where epoxy won't spray in well.  Then a coat of epoxy, and seam seal the cracks on top of the epoxy.  Then paint/undercoat.  Then do the insides of the rails with Eastwood inner frame paint.  Then, if I feel really anal, SEM cavity wax on top of that.

 

MIght be a bit much...but none of these steps are particularly expensive or time consuming...figured I might as well do all I can to prevent future problems.  As was stated earlier, these cars have so much layered metal and moisture traps, it's a wonder they haven't all rotted away to nothing.  I have noticed in many places though, that the factory spot welds were done right through the factory yellowish primer.  It looks nothing like standard weld-through primer, so I have wondered what sort of formula it was.  I have very little faith in rattle can weld-through primer.  Even though it is "weld through", it is so darn fragile I think on balance you are better off just using epoxy and cleaning it off the small areas that need welding.

 

There is no perfect answer.  So I probably won't drive the car in the rain....LOL 

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  • 10 months later...

Check this YouTube video.  Looks like a very well thought out test.  I'd love to have seen more products tested such as ZRC, Rust Bullet, Kryptonite, etc.

 

Many years ago (around 1996 maybe) I tested the lot of rust preventers on the market, including latex-based rust converter, Eastwood Heavy Duty Anti-Rust, ZRC (Zinc-Rich Compound) and Corroless.  The test was fairly simple.  I started with clean sheet metal (1012-1018) and coated each piece as directed, except I did not coat the edges as I wanted to give the corrosion a "path" to the faces.  After curing, I scratched across each face with a sharp tool to cut through the coating, then sank the pieces into a sink with salt water in it, and kept the salt water fresh by changing it out every couple of days.  After a few weeks, the ZRC-coated steel was the only one that had not categorically failed or rusted.  Most of the coatings lifted from the metal (that started as bubbling - not intrusion of rust from the edges) and began to rust where the surface became exposed to the salt water.

 

https://www.youtube.com/watch?v=lyWHF4NoNVk

 

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For panels that overlapped in my car that needed welding I've used a zinc based weld through primer.  My only testing was on some metal I removed from the car, sanded down, sprayed with the weld through primer, and welded up.  I left these outside for a couple of years.  They had the usual surface rust but nothing major on the weld areas.  This isn't much of a test but that's what I've done for overlaps.  This will be for a race car that is mostly pampered and will only get wet in occasionally.  

 

What Ironhead describes seems to be how most of the OEMs do it now.  You need somewhere for trapped water to go and use a lot of the new seam sealers.  Ever take a new car apart and notice how much of that stuff is everywhere?  OEMs aren't into spending money on things not needed so that's probably a good thing to follow.

 

Hope this helps,

Cary

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It seems the consensus is to use cavity wax as the primary (last step) moisture-proof sealer, which makes very good sense.

Of course, the issue is what to do before that.  Since my car is on a rotisserie, I can orient it as needed to position certain openings 'down'.  Then flood the cavities with ZRC until it runs through any openings between welds, sealing them against moisture and offering sacrificial protection .  After lots of cure time, finish with cavity wax.

The problem with ZRC, like any zinc product I guess, is most paints don't do well as a top coat, so you have to remove any that makes its way to a finish surface.  Small price to pay for the protection afforded inside the dark recesses of these cars.

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  • 9 months later...
  • 1 month later...

On my current build I used a total of seven cans of 3M cavity wax, inside all the frame rails and anyplace else where there is "blind" metal.  Seven cans sounds like a lot but it goes pretty fast.  This stuff really does seem to run into even the smallest cracks, and after applying it you will find it running out everywhere there was the slightest gap.  I even applied a couple of coats inside the "A" pillars, because I tied the cage to them and the welding certainly burned off whatever was applied inside them at the factory.

 

It can be endlessly debated the merits of weld through primer, epoxy, seam sealers, etc in the areas to which we have some level of access.  However, there are many, many places on these cars wherein cavity wax applied with a "wand" is really your only option for corrosion protection.  There is simply no other way to get at some areas. 

 

Just make certain you are 100% finished with painting and finishing before applying these waxes.  It is quite a job to clean off adequately to allow paint....or anything else....to stick to surfaces they are applied onto.

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