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Old 05-24-2012, 07:27 AM   #1
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1981 31' Excella II
New Market , Alabama
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Attaching Lower Skins with Bolts

I am finally getting around to putting the skins back on the rear section of my trailer that I replaced the last 4 feet of rear floor on last fall. I am wanting to use screws for easy access and inspection of the floor area. Since mine is a center bath there is only about 6 feet of skin that I will be dealing with. On 2 of those feet between the rear most tank and and the mounting plate for the BAL jacks I plan on keeping the existing skin. So lucky for me that leaves 4ft which is the width of my 4ft x 12ft 6061-T6 sheet of aluminum. I can get just about all the skins in one panel. I will be replacing the outer curved skins as well and I will be making a skin for the rear where the EVIL BUMPER plate use to be. Water will no longer be able to get to my sub floor via that plate nor will it be able to get between the floor and the skin. About 90% of my frame rust issues were due to water that came from the bumper compartment and that flat plate. The bumper compartment is cleverly designed so any water that does not end up in your sub floor will go underneath and rot out your frame.

The issue of what type of bolts to use on aluminum has come up. Stainless steel will cause corrosion of the aluminum since aluminum will be the anode and stainless will be the cathode. I don't want to use aluminum bolts because they will corrode before the frame does and fall apart. Zinc plated bolts are ok but the zinc coating does not last long. Zinc is anodic to steel, stainless steel, and aluminum. However, knowing this I can use a double dip galvanized (really thick zinc coating) flat washer under a stainless steel screw. This should protect all three metals by becoming an anode or at least not making the situation worse. I know I am probably going to get some flak on this. I am open for suggestions.

The Airstream approach to using aluminum rivets in steel does not seem to work. The sheet aluminum corrodes right around the rivet and comes loose as all of us know. Coating the frame with POR15 to reduce electrical contact with the frame is a step in the right direction. You are going to break the coating with any type of fastener you use.

Not having the bumper plate and the stinky pink stuff is going to help a lot just by itself so maybe I am overthinking things.

Perry
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Old 05-24-2012, 08:08 AM   #2
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1977 31' Sovereign
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The stainless usually forms a passive oxide coating and does not cause that much corrosion. If you want to use the hot zinc dip washer, they will act as a protective anode for both the stainless and the aluminum. We used that approach, when we installed zinc anodes on all the Johnson and Evinrude outboard motors. I was their material engineer and corrosion engineer. The washers will need to be replaced whenever they show signs of rusting, indicating the zinc has been depleted.
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Old 05-24-2012, 08:11 AM   #3
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No flak from me. I used to work on Al skinned "dry" kilns that are made from Al and stay hot and wet and slightly acid while in service. A slight internal pressure forces everything into the walls. Everything corrodes. I would use stainless for anything structural and let the Al take its chances. I would probably just use the Al rivets like Airstream does to put the interior skins back on. Everybody jumps on Airstream, but they do have a lot of longterm experience with how they do things. I know on the dry kilns everybody had a better idea for insulation, skins, fastners, etc. And many of the new ideas ended up in a lawsuit. The thing that is going to help the most is keeping the water on the outside.
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Old 05-24-2012, 08:58 AM   #4
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Well at least I am on the right track. I think I will thread everything for 1/4-20 screws just like it is from the top.

Perry
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Old 07-06-2012, 09:13 AM   #5
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I am having the same dillemma at the moment and have decided onzinc coated nuts and bolts with stainless washers? will have to be better than just steel right?
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Old 07-06-2012, 10:33 AM   #6
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I used stainless screws and Zinc plated washers. The Zinc is anodic to both the stainless and the aluminum so it should protect both of them. Zinc plated screws will eventually rust and seize where stainless won't do that. Not a bad idea to use antiseize on bolts just in case. The washer will have more surface area than the bolt.

Perry
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Old 07-06-2012, 11:12 AM   #7
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I missed this thread eariler, but I used Cad plated bolts and aluminum washers.
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Old 07-06-2012, 02:02 PM   #8
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How long you think the Cad plating will last Kip? The Aluminum washer is a good idea. Just don't put those Cad plated bolts in your mouth. I am not looking forward to messing with the forward skins. The .032 thick aluminum is perfect for the bottom skins but a little stiff for the side wraps. That stuff is stiff enough it does not take that many fasteners.

Perry
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