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Old 07-01-2015, 10:13 AM   #1
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Heavy corrosion: metal prep?

got some heavy corrosion on the bottom 2" of the rear panel. I'm going to be putting in a doubler, riveted to the solid skin a few inches above. In the mean time, I'm wondering how to deal with the existing corrosion to keep it from getting worse. (maybe it doesn't really matter, since I'll be using a doubler, but it seems as though I should do something).

Anyway, I've seen people suggest the aircraft solutions, alumiprep and alodine, but they're kind of spendy, the smallest available containers have probably 10x what I would need. I got some por-15 metal prep for doing the frame; they say it can be used to prep aluminum for paint, as well, so I'm wondering if this would do a similar job. Their docs say it leaves a zinc phosphate coating. Then I'd hit it with some epoxy primer.

The problem is really only on the skin; the U channel only had some very minor pitting. Since the legs on the U are vertical, and the hold-down plate is angled, the 2 pieces don't really touch; but the back-side of the hold down plate is flat-up against the skin (for much of its height), and the area back there is really bad.

any thoughts appreciated
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Old 07-02-2015, 12:53 PM   #2
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For the amount of work to install the doubler you can easily replace that skin. The big issue is that if you don't completely remove the corrosion it will continue to corrode between the C channel and the exterior skin. Do you have the Galvanized angle in there also, because it is also most likely causing the corrosion that is creeping up the skin. if you look at the beginning of my thread below you can see the same issue that I had in the same area.
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Old 07-02-2015, 09:04 PM   #3
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It was galvanized?
didn't look it.

I took it out a couple of days ago...it looked like half the thickness of the skin was stuck to the back of it.

If I were to replace the whole panel, I'd have to re-do the "A I R S T R E A M" letters, and all that...the finish wouldn't match, etc. seems like a major pita.
On the outside of the skin, the corrosion is limited to the area that was covered by the belt line trim. Above that looks like new. clear-coat and all.
On the inside, the same bottom 1 1/2 inches, plus a little more where the hold-down plate was touching.
I prepped the frame for painting today. The label on the metal-prep says its phosphoric acid...leaves a zinc coating.
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Old 07-02-2015, 09:08 PM   #4
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Here's a couple of pics from the outside:



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Old 07-02-2015, 09:13 PM   #5
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Chuck,
I think I'm having deja vu.
I could swear that you asked this question a year or so ago.

Anyway, replace that sheet.
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Old 07-02-2015, 09:28 PM   #6
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here's what the hold-down plate looked like when I pulled it out:

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Old 07-02-2015, 09:43 PM   #7
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The rear lower aft skin and structure is the major attach point between the shell and the frame, this area is where the rear separation all starts. If your intent is to still splice than at least do it properly. Cut the entire bottom of the skin off to good metal. The doubler should be one gauge heavier then original, with the doubler picking up two rows of rivets on the new and old skin alike. The rivet spacing should be 6 to 8 rivet diameters apart and you should only use AD rivets. Use a quality sealant and shoot it together wet
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Old 07-02-2015, 10:29 PM   #8
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Quote:
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Chuck,
I think I'm having deja vu.
I could swear that you asked this question a year or so ago.

Anyway, replace that sheet.
I'm sure I probably did, and just don't remember.

Oh, well...I suppose if I just cut off the bottom 1 1/2", then I don't really care about the corrosion, right?

Its amazing that the thing was holding together as well as it was. Those rivets were very firmly seated. The skin is soooo brittle, there, though--hard to believe that the skin hadn't cracked around the rivet heads.

Kip: what are AD rivets?

You know, there never was any rear end separation on this one. The bolts going through the hold-down plate were all still intact. Heavy rust, but all still there. there was no movement in the frame at all. The floor had disintegrated long ago...but everything else was there.
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Old 07-03-2015, 11:13 AM   #9
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They are a tempered rivet with an alloy number of 2117, they have a greatly increased shear load over the "A" rivets used in the shell. I believe that your trailer should already have them installed in the "C" channel to shell interface
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Old 07-03-2015, 11:18 AM   #10
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Paint everything well on the inside to insulate the different metals. The aluminum is an anode to the steel. That is why the aluminum is Swiss cheese and the steel is ok.

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Old 07-03-2015, 05:17 PM   #11
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hold down was rusty and pitted, but it'll be fine.
Heading out to put the second coat of por-15 on the frame now.
(what a p i t a that is).

do you think "paint" is enough for the hold-down plate? (for insulating the metals). or is there something more/better? rubberized undercoating, etc?
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Old 07-03-2015, 05:48 PM   #12
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Chuck,

I would brush on a layer of Vulkem or another polyurethane sealant as a barrier between the steel and aluminum.

Paint is probably more than what Airstream did in 1973.
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Old 07-03-2015, 06:33 PM   #13
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POR 15 is pretty good stuff. Won't hurt to wet install rivets with the stuff either. Dip thi rivet in paint then install and buck. Do this on the hold down plate area only or you will have a mess. It will be messy anyway.

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Old 07-03-2015, 07:56 PM   #14
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Second coat of POR is on. what a sucky job that is. I hate painting.

yeah, it didn't look like the hold-down had any paint on it. The side that faced in was just bare metal on the top...not even any rust, where it wasn't touching anything.
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