Prime?

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Pdxsling
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Prime?

Post by Pdxsling »

Just curious how many builders have decided not to prime? And why? I’m starting sling 4 and have gone back and forth. Thank you.
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PhilipRueker
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Re: Prime?

Post by PhilipRueker »

It's an endless discussion, so if you ask 100 builders, you will likely get 101 opinions.

The high level summary though is, that the Aluminum used in the Sling is 6061-T6, which is already pretty corrosion resistant and so you can likely get away with not priming and will be fine for many years to come.

That being said, the factory does prime all mating surfaces on their builds (and quickbuilds) and suggests builders to prime the mating surfaces.

I for myself have decided to prime the mating surfaces of all the exterior Elements.
I'm using a simple self-etching primer - some of the background on my decision and investigation was that I ended up using the same primer as Matthew Harris - Rustoleum Self Etching Primer and it seems to do an ok job if you prepare the surface accordingly, but it will add to the build process since the prep takes some time.

Some other considerations you could have of course is where and how you are planning to store the plane after it is done - like do you live directly on a salty coast or other consideration can be whether you are going to keep the plane in a Hangar or outside.
Building a Sling TSi in my Garage.
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ebrunye
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Location: San Diego

Re: Prime?

Post by ebrunye »

I started off like Philip, using the home depot rattle can primer for my empennage. After more research, the prime/no prime debate is caused by almost all aircraft being constructed out of a different kind of aluminum. 2024 is what Vans and Cessna use, and is somewhat an industry standard for "aircraft grade aluminum". Mostly because of strength/weight ratio. However, 2024 is only about 93% pure aluminum, with the main alloying element being copper (about 3-4% of the alloy) and we all know what copper does when left in the elements- think statue of liberty. Pure aluminum is phenomenally corrosion resistant. It basically creates an oxide layer on the outside immediately when exposed to oxygen, which effectively protects the base material. This is why Vans uses a layer of pure aluminum to coat their parts, called Alclad. Alclad would be all that's needed, if you could be sure it never scratched off, which you can't be.

Enter 6061. 6061 is 98-99% pure aluminum, with the main alloying element being magnesium, and its typically less than 1% of the total alloy. 6061 is commonly used to create aluminum boat hulls, so you see it's a good contender against even salt water. Boats that use other alloys (mainly 5052 or similar) aren't doing it because of the marginally improved corrosion resistance. They are doing it for weldability. 6061 has poor welding properties, which is irrelevant to this discussion, so I digress. 6061 is very slightly lighter in weight than 2024 that most other aircraft use, but is actually somewhat significantly weaker (about 30-40% in tensile and shear!) So this makes the strength/weight ratio much less attractive than 2024. However, the design of the aircraft can take this into account and be compensated with more or thicker ribs, thicker spars, thicker skins, more rivets, etc. The added benefit of 6061 is slightly cheaper and easier to obtain, and is absolutely in a different class when it comes to corrosion. It won't corrode, that shouldn't be a concern, except in the case of galvanic corrosion which occurs when dissimilar metals come into contact with each other. For example, the main spar carry through on the fuselage uses stainless steel rivets. These will require some corrosion protection, which the manual mentions, so you don't end up with ions flowing between the metals. This is why boats use what's called a "sacrificial anode" basically a chunk of magnesium or zinc, to take the ion hit instead of your precious aluminum frame.

I'm not aware of any aircraft manufacturer who used 6061 40 years ago when everyone was busy using 2024. If anyone knows of one, I'd be interested to look into how those particular planes are holding up to corrosion 40-50 years later. Even unprimed cessnas that are 50 years old are still out there flying just with some corrosion-x treatments etc. after sitting outside tied down their whole lives.
Pdxsling
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Re: Prime?

Post by Pdxsling »

Thanks for the info guys. It’s a hard topic with so many different opinions. I don’t think I’m gonna prime, my workspace just isn’t ideal for priming and the sling will be hangared. Still a tough decision, anyway thanks again for the replies.
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ibgarrett
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Re: Prime?

Post by ibgarrett »

Evan,

Just for my clarification, so when you are mating the stainless steel and the aluminum you'll need to put a touch of corrosion protection on the rivet?

Thanks - Brian
Brian Garrett
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PhilipRueker
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Re: Prime?

Post by PhilipRueker »

ibgarrett wrote: Mon Aug 03, 2020 8:24 am Evan,
Just for my clarification, so when you are mating the stainless steel and the aluminum you'll need to put a touch of corrosion protection on the rivet?
Thanks - Brian
Yes that would be the idea. You can read the Service Bulletin 0014, which asks to replace some rivets on the wings with stainless steel rivets and it suggests to dip the steel rivet in corrosion prevention and then rivet it.
Building a Sling TSi in my Garage.
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