Is this corrosion or grease/grime on interior skin?

[Jeremy9107]

Posted this in our reno thread also.

Does anyone recognize these blemishes? They are on the inside of the outer skin. Photo 1 is along roof line. Photo 2 is skin that wraps under belly. What is best method of stopping/removing corrosion? Or removing grease?

[overlander63]

That's just "stuff" that was sprayed on the inside. When new, it was sticky, and things like insulation and wiring clips would be glued to the inside of th epanels with it.

[perryg114]

Yes there is corrosion there. The second one is the worse of the two. I would not worry about removing it. You can coat it with POR15 or some stuff called Aluthane. Best way to protect it is to coat it and keep water away. Don't put the stinky pink insulation back in. With that side skin the way to keep it from corroding is to electrically isolate it from the steel frame and also make sure it can stay dry. One way to insulate it is to paint it. Any salt spray should be washed of immediately. Best to not drive it on salt covered roads.

Perry

[Jeremy9107]

Just a follow up :

Used Alumiprep 33 on inside of skins to clear any possible corrosion. Followed factory instructions and diluted 1:5 on light corrosion and 1:2 on heavy corrosion. Applied for 3-5 minutes, kept wet, scrubbed when necessary and thoroughly rinsed with water. It worked great on those skins that wrap under the belly. Basically all the discoloration is gone. It did not do much for those circular spots on the ceiling so assuming that is not corrosion and is just "stuff" that also resisted removal with degreaser and Xylene.

[perryg114]

Keep in mind aluminum will self anodize (corrode) and protect itself from further corrosion. A little patina on the inside surface won't hurt anything. The white powder is a problem and more serious. Aluminum has issues when it is contact with other metals, exposed to salt, or kept wet all the time.

Perry

[Wabbiteer]

Sorry I'm late to the party - Alodine, chromic-passivation treatment while still fresh from the Alumaprep, would be the aircraft quality metal treatment -- but this stuff is perhaps too great of a hazard in itself for the Sunday hobbyist (Cr(III) good, Cr(IV) bad, etc.).

A weak POR-15 Metal Prep wash now* to get oxidation susceptible metal surfaces protected might be ideal - However, that clean bright surface on the Airstreams shell interior surface likely will not a liability for about the next thirty years since it is a conditioned or protected living space and not a tanker truck or saltwater motor yacht...

Don't forget a powerful leaf-blower does wonders chasing water films out of crevices....

[Jeremy9107]

Would Klean-Strip phosphoric metal etch be a suitable substitute for POR-15 metal etch as the post Alumiprep 33 wash you are suggesting? Have plenty of that laying around.

[Dingo Girl]

Originally Posted by overlander63

That's just "stuff" that was sprayed on the inside. When new, it was sticky, and things like insulation and wiring clips would be glued to the inside of th epanels with it.

Looks just like my inside of the outside walls looked. It was sprayed on. I left it.

[perryg114]

Coat it with something like POR15 especially where the rivet holes are. There is a lot of corrosion there where it was connected to the steel frame and kept wet all the time. The best way to prevent corrosion is to insulate and coat things that are in electrical contact or exposed to water. POR15 loves a ratty rough surface to adhere to. You don't want to put it on pristine metal. An alternative is Aluthane. Both should get the job done. The yellowing/gray is good. The white powder is bad.

Perry

[Wabbiteer]

perryg114 said aluminum is naturally protected and that's true with proof being 50-year-old trailers lounging around outside as we read this...

Yet, make aluminum into an alloy and things change, thus the alclad pure aluminum coating on vintage trailers making them near as immortal as lawn-furniture <cough> can be. Later Airstreams w/o Alclad had other alloys but with etched/treated surface under clearcoat applied before it reached Airstream, works well, lasts long time, cheaper too.

Klean-Strip phosphoric metal etch is 35% H3PO4 or acid - you've already etched any oxides or odd metal salts off the sheet aluminum with the Alumaprep-33... Yes, phosphate treatment builds a film on the aluminum surface but it is not a reliable anti-corrosion layer.

Hard tap water, most degreaser cleansers, most anything will react with freshly etched raw aluminum and contribute for better or worse. I wasted $30 of chromic treatment by soaking sheet aluminum that the degreaser I’d used after the acid (good intentions gone bad) had ALREADY blocked any new adhesion. Oops. Simple Green Extreme is the only degreaser to be around raw aluminum prior to surface coating treatments.

For aluminum, since any exposed surface burns, err oxidizes immediately on exposure to oxygen, they quickly use dissolved metal ions in phosphoric as carrier/helper to shuffle other less reactive metals like chromium or zinc (comparatively) into the aluminum surface & pores. There is also a time limit on how long the follow-up treatment can be from the prep etching.

On clean aluminum interior seam sealing... butyl rubber gutter caulk is about the closest match to the old vintage factory caulks used, with todays reduced VOC regulations the old good stuff is impossible to find unless its special purpose like on a rail car factory assembly line or something.

Tremco is full of silica grit for better metal-masonry adhesion in building construction, it sets fast-ish and is hard to force into a thin bonding/sealing layer between sheet metal layers. The Sikaflex, 3M5200 and other modern RV/Marine caulks are awsome but spendy. Just to parge over rivet lines either the Butyl or Tremco would rock.

Now is the time to note the long-sheet Airstream shell panels, e.g. roof sections, came pre-punched for rivets and often they didn't line up so about every 40 or 50 there is an empty hole inviting water intrusion, now it's search and destroy time while you have the liners out. Forum member Darkspeed pointed that out when he dropped in to visit when he was going cross-country way back when.

[Jeremy9107]

Wabbiteer. Your advice always leaves me with more questions, but keep it coming!

"There is also a time limit on how long the follow-up treatment can be from the prep etching."
What is that time limit? Wasn't able to find POR15 metal prep in box stores. Used it to prep frame but ran out. Company rep said phosphoric acid would work as substitute so bought half gallon of it. If time limit not up, will order more of the POR15 on Amazon now. Do you apply same way as if you are prepping to paint?



"Tremco is full of silica grit for better metal-masonry adhesion in building construction, it sets fast-ish and is hard to force into a thin bonding/sealing layer between sheet metal layers. The Sikaflex, 3M5200 and other modern RV/Marine caulks are awsome but spendy. Just to parge over rivet lines either the Butyl or Tremco would rock."
Already bought Sikaflex 221 intending to use on interior seams and rivets. Think that was something picked up in old thread of yours.

[perryg114]

You don't need the metal prep. I put some aluthane on section of untreated skin and it adhered like mad. I was skeptical as well but it sticks. Call these guys.

http://www.epoxyproducts.com/aluthane.html

Perry

[Wabbiteer]

You don't need further anti-corrosion treatment. Besides, it might get in the pores of that sexy floor you installed and be an unwanted presence until the end of time.

I'm on the periphery of the Sport Aviation crowd, if there is bare aluminum it gets a chromate conversion coating without exception, usually followed by high-dollar primers and paints.

Airstreams are not aerospace engineering but for another $35 'protection money' I gave interior spars, ribs, rivets and shell the best chances I could. If you were doing spray-foam or a paint-on insulation layer I'd be cheering for a zinc/zircon/chromate process so the bond would be maximized for the next half of forever.

The time-limited application window is/was to get the most thorough chemical reaction results from juggling more nasty chemicals after the Alumaprep steps. You already wiped it clean, no molds or mildews there now, no 38 pounds of bacon cooking fume residues, no black tank belches hiding anywhere... good job.

Any form of Chromate in phosphoric becomes Chromic Acid, chromates are supreme aluminum treatment as unreacted trace amounts left in/on aluminum will not corrode. I think* there are whole religions built around the varying processes...

Alodine 1201 is the standy-by, Alodine 1001 is a colorless process; 1201 has a orange tint for judging treatment quality but could turn your window frames golden. I ended up using both, I'd planned on paint & DIY foam kit for the first 1/2 to 1-inch of insulation. For education purposes only here's links to vendor that doesn't gouge for hazmat on the gallon sizes...

http://www.aircraftspruce.com/catalo...lodine1201.php
http://www.aircraftspruce.com/catalo...lodine1001.php

[Jeremy9107]

1) The shiny appearance of the skin is rather pleasing compared to the gunk that was on it before (and you said it Wabbiteer, the smell that we have been guessing origin of for past 5 years is gone too).
2) Need clean surface to stick foam board strips and Protecto tape to for next step of insulation install (using bubble foil).
3) Spent so many hours scraping nasty stuff off and inhaling toxic fumes from Xylene would hate to just cover back up with paint. So would pass on your idea Perry with the Aluthane and thus am assuming the Alodine is not needed.

Mostly wanted to be sure was not missing a necessary 2nd step after the Alumaprep.

Turns out have to take shell off again to figure out those wheel tubs so would not effect that pretty floor even if needed to use Alodine. Echo comment on Aircraftspruce. They had Alumaprep at doorstep in 2 days without big hazmat fees!