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Permalink Reply by Bob Simmons on July 9, 2021 at 10:49am

I'm curious about this as well. My 750 cabin frame hung from a rafter in my workshop untouched for years after I broke my back. It wasn't exposed to the elements other than humidity in a space that isn't climate controlled (mostly in Ohio). It has some superficial surface corrosion, but nothing structural, and I was planning to have it blasted and powder coated. But that got me thinking. If surface corrosion eventually will form on the surface of the tubing, what happens to the inside of the hollow tubes? My experience in the Air Force taught me that, while superficial surface corrosion isn't a structural concern, it can become such a concern if left untreated for long periods. So has anyone ever thought to consider what happens on the unpainted inside of the tubing over the long term? Powder coating doesn't cover the inside, so is that a concern? The ends of the tubing are not capped in any way, so the environmental effects can apply to the inside of the tubing as well as the outside. Is there a way to mitigate the problem if such a concern is real? And for me personally, should I consider ordering a new cabin frame since mine sat untreated for so many years? Maybe I should just ask Zenith, and figured I probably would, but I was curious if anyone here had considered this stuff.

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Permalink Reply by Kevin Powell on July 9, 2021 at 10:59am

Well, that's a good point!

I'm wondering if plugging one end of the open tubes and pouring Corrosion-X into the tubes & letting it sit for a while before pouring it out would be worth doing? 

Then the question arises: what would the impact of this be (if any) during a subsequent cleaning/coating/baking process be? Could that residue inside the tubes vaporize and affect/contaminate the powder coating?   

If I was just going to abrasive blast, prime and paint the frame myself, I still think I'd go ahead and try to treat the insides of the tubes with Corrosion-X.

Going to a "highly recommended" abrasive blaster today to get his input about how to prep the frame. Maybe he's got insights about treating the inside of the tubes....

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Permalink Reply by Bob Simmons on July 9, 2021 at 12:36pm

Had someone else tell me an AD was put out on some high wing Pipers for this very problem, tubes rusting from the inside out. The practice now is usually to fill the tubing with linseed oil, drain it, and weld the pipe ends shut. Alternatively, Vans tells their builders to pour corrosion inhibiting paint into their pushrod tubes, so that could be a possible solution here, like you said.

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Permalink Reply by John Austin on July 9, 2021 at 4:26pm

In some of my 4130 tubing, such as the spar carry-through, I sprayed the interior with SEM Rust Preventer Cavity Wax.  I used a flexible SEM "Aero Wand" with the spray can to insert inside tubes and spray a 360 degree fan pattern.  I think the Aero Wand has been discontinued and now they have a 36" rigid spray wand (which I think will flex a slight amount) that also sprays a 360 degree fan pattern.

John

N750A

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Permalink Reply by Kevin Powell on July 9, 2021 at 4:31pm

Thanks John. I like SEM products (use their grey primer on my project).  I'll look for the Rust Preventer Cavity Wax.

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Permalink Reply by Kevin Powell on July 9, 2021 at 6:51pm

So, a follow up question about that SEM Rust Preventer Cavity Wax:

Did you use SEM solve or something else to prep the surface before application?

Did you use more than one aerosol can to treat the inside of your cabin frame?

And, it's still possible to buy the "AeroWand"; where you able to clean yours up when done or just toss it? (The RPCW appears to be pretty sticky)

Lastly, I assume you applied all this stuff to the inside AFTER you primed painted/powder coated the outside of your cabin frame, right? (BTW, how did you finish your cabin frame?)

Thanks, John.

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Permalink Reply by John Austin on July 10, 2021 at 6:49am

I cleaned the cabin frame with acetone, then scuffed with Scotchbrite, another acetone wipe-down and 2 part epoxy primer (Rustoleum makes a true 2 part epoxy primer in a rattle can, available at industrial supply houses, so line up a bunch of parts and use it all up once it is activated!) and paint topcoat.  

I discovered/used the SEM wax after the cabin frame was painted, and only used it in a few places such as the spar carry through, etc.  I discovered a mud dauber nest in the spar carry through so I poked it out, swabbed the interior of tube with Simple Green Extreme solution to remove last traces of the nest, then swabbed with acetone and sprayed with the SEM.

I flushed the Aero Wand with some solvent (I think I used brake cleaner or carb cleaner) and reused it several times.

John

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Permalink Reply by Kevin Powell on July 11, 2021 at 12:40am

Thanks for the additional details, John. I 'm leaning toward using the SEM Rust Preventing Cavity Wax. Will check into the Rustoleum rattle-can 2 part epoxy primer.

Still thinking hard about having someone do an abrasive blast on the frame first, just to ensure the light bit of surface corrosion is all gone.

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Permalink Reply by James Neirinck on July 9, 2021 at 7:14pm

Exterior of frame painted with automotive 2 part epoxy paint sprayed in booth with HVLP gun.  Prep was to scotchbrite clean the tube and then wipe with solvent, prime with zinc phosphate primer.    For inside of tubes used boiled linseed oil warmed up and poured inside tube.  Capped ends with rubber stoppers and rotated tube / frame to ensure oil fully coated all of the inside of the tube.  Then uncapped and poured out excess linseed oil back into the container.  Let sit for a few days as the linseed oil will become the consistency of wax.  Note that the oil will continue to drip out for a while so either recap the tube or leave it with the open end sitting up or at least a bit higher that the rest of the tube.

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Permalink Reply by Kevin Powell on July 11, 2021 at 12:42am

Thanks for the input, James. Seems like either linseed oil or the aerosol/sprayed cavity wax is the way to go. I didn't realize linseed was that thick (as purchase).... or, does it become that way as a result of boiling it for a while? 

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Permalink Reply by Oliver Reik on August 1, 2021 at 7:43pm

Like John, we used SEM Rust Preventer Cavity Wax. Instead of spraying, we however heated it up in a hot water bath, poured it into the frame and then moved the frame around, so that the hot wax gets everywhere.

Afterwards, we drained the excess wax.

We did the same with the control tube.

You want to do this only AFTER painting the components.

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Permalink Reply by Kevin Powell on September 13, 2021 at 3:16pm

Thanks for the response, Oliver. 

Sounds like you purchased the SEM Rust Preventer Cavity Wax in a can (non-aerosol). I purchased it in a spray can and bought the "aero" wands... so looks like I'll be spraying it. Will definitely ensure I prime/finish the outside surface of the cabin frame first. I also never gave much thought to coating the inside of the control tube, but makes sense. Thanks for the insight.

Kevin

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