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Damaged dimple on a skin, is it fatal?

romanov

Well Known Member
Hi Guys ,

I need your educated opinion here, we damaged a
dimpled holes a bit, what do you think is it so
bad that requires a new skin?

here is what happened:

We just started to scuff the VS skin (VS-801PP)
as a preparation for priming / coating.

Accidentaly we applied too much force on the inner side where dimpled cone is.

As a result there are a few dimpled holes that are a bit larger now: (about 0.15mm) and they are a bit flatened , you can see the results on pictures attached. Also
have to add here that the front side of the holes is
completely fine.

Please advise if it may cause a problem or may affect the structure strenght.

The big question now is , whether we can continue with that skin or we have to order a new one.

Looking forward for your recommendations.


check that album for illustration:
https://imgur.com/a/Oes7Ocq


Blogging my RV14 here:
https://bit.ly/2PzjPzS
 
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The general rule is that you can adjust a dimpled hole one time, either reversing it (if you dimpled the wrong direction) or re-dimpling. After that, the aluminum is considered fatigued and needs to be replaced.

I would try to dimple them again first, but just once. If that doesn't fix it for you the best solution is to just get another part and consider that one practice .. I had lots of "practice" :D
 
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Vans is the final say, but these aren't 'reverse dimpled'. I'd say touch them up with dimple dies again and let the rivet do its job, maybe upsize one length rivet.

EDIT: The heavy mechanical scuffing method may lead to more issues and would change my recommendation.
 
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Personally I would go to 1/8" rivets in these holes. You are using way too much scuffing force. Very little scuffing is needed, especially if you are using a self-etching primer.
 
Thanks guys, the mail to Vans been
sent. Your ideas are very interesting.

How often people go to 1/8 rivet
while killing 3/32 holes?
 
How the heck did you scuff the skin hard enough to both flatten a dimple and enlarge a hole? How is this even possible? What are you using to scuff ?
 
Oops Rivets..

..have the same head as an AN-3 but the shank of an AN-4. However, if this condition is over a whole lot of holes, I'd start again, since you've hardly begun.
 
...and

And remember, if you need to go up a shank size, you can use "Oops" rivets. They have the same sized hand as the specified rivet but the shank is one size larger...
 
..have the same head as an AN-3 but the shank of an AN-4. However, if this condition is over a whole lot of holes, I'd start again, since you've hardly begun.



That one. Wont do it again.
https://imgur.com/6j97R5H

6j97R5Hr.jpg
 
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I don't think you've flattened the hole, but actually ground it down. Thats why you're seeing the larger hole. As the skin would now be pretty thin around the hole, I would replace the skin if there was more than a few. I'd suggest just scuffing by hand next time.

Alex
 
Oops rivets are not likely going to work as the hole is now too large. Larger rivets might work but unless you do all the skin with them it is going to show.
As this is you first part I would replace the skin. Keep this skin for parts if you need some small pieces of aluminum later in the build. Set this part aside and move on to something else you can get a skin shipped with your next order to save some shipping costs.
Scuffing should be done by hand with a red Scotchbrite pad. Very little is required, just enough to see some light etching.
I see that you are based in Israel. You might consider not scuffing and priming as pictures of your country would lead me to think it is very dry. However I do not know the effect that Mediterranean has on local cars and equipment. Go and talk to a local aviation shop and ask their advice on priming.
 
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Personally I would recommend looking at a new skin. Based on your picture you were using some decent sand paper with your orbital sander. I would highly recommend a visit from an EAA Tech Counselor or possible the EAA Sheet metal class.

I would not recommend sand paper to scuff before priming, but red scotchbright pads. Also as others have stated, Scuff before you dimple (I would go farther and say scuff and prime, then dimple after primer has fully dried). If you REALLY want to use your orbital sander, just remove that sand paper and put a maroon scotchbright pad on it and use LIGHT pressure. You are just trying to scuff, not sand any thickness off.

Maybe give us a photo of the area so we can get a better idea. Could be it just sounds worse than it is and OOPS rivets may be fine, but hard to tell.
 
An old-timer I know and respect a whole lot told me that virgin alclad does not need to be scuffed. You're either going to use a conversion coating, and primer. Or, you're going to use a self-etching primer. With clean, new alclad skins and extrusions, just wipe with acetone using a lint-free cloth to remove ink and oils. Then, wipe again, just before priming with denatured alcohol.

He was emphatic that scuffing compromises the alclad and is just not necessary, unless the area is already compromised by salts or corrosives (like when you find corrosion under the blue plastic around the edges of alclad that has been stored a long time).
 
The goal of the scuffing is to provide a key for the primer to help it 'etch' into the surface. This only requires a quick light scuff by hand with Scotchbrite as others here have mentioned. Just enough to lightly scratch up the surface - not so much that all of the shiny Alclad layer is removed. The shiny Alclad helps prevent corrosion, so you want to retain it, but you also want the primer to stick to it.

PM me if you would like to Skype and I can demonstrate a few things for you - I know the feeling of building in a country where you can't just get another builder to come over and have a look.
 
alclad does not need to be scuffed

The parts definitely need to be scuffed and cleaned with Acetone or MEK to provide an oil-free non-smooth surface for a primer to grab onto, otherwise it's going to eventually flake off.
 
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If you REALLY want to use your orbital sander, just remove that sand paper and put a maroon scotchbright pad on it and use LIGHT pressure. You are just trying to scuff, not sand any thickness off.

Would an interface pad help as well? I've used them cutting clear on cars and they really save you from cutting through sharp corners and following curves. You end up with a lighter touch than by doing it by hand.
 
+1 on Brian Lester's comment

I've used a quarter sheet finish sander with the red scotch bright pad (instead of sandpaper) with great success. I always tried to scuff before dimpling, but the times I've forgot the thickness of the pad and very light pressure (not much more than the weight of the sander) allows the pad to conform to the dimple without any material removal.
 
Random orbtal

I've used a quarter sheet finish sander with the red scotch bright pad (instead of sandpaper) with great success. I always tried to scuff before dimpling, but the times I've forgot the thickness of the pad and very light pressure (not much more than the weight of the sander) allows the pad to conform to the dimple without any material removal.

Same here. Maroon scotchbrite on the pad of a random orbital. I just guide it around with no pressure very quickly.
 
Consider changing the process. Scuff before dimpling.

+1

When I built my VS I dimpled everything before I primed. I'm glad I did it then to see how much of a pain it was to scuff those dimples. I hand scuffed with maroon ScotchBright pads and found to get an even scuff around the dimples I had to swirl the pad around them to touch the entire surface. If I used a back and forth motion I would leave spots on the forward side of the motion untouched.

After that experience I did my surface prep and prime before any dimpling. I'm using Akzo two part epoxy primer and it's tough as nails. It'll handle the dimpling without problems.
 
An old-timer I know and respect a whole lot told me that virgin alclad does not need to be scuffed. You're either going to use a conversion coating, and primer. Or, you're going to use a self-etching primer. With clean, new alclad skins and extrusions, just wipe with acetone using a lint-free cloth to remove ink and oils. Then, wipe again, just before priming with denatured alcohol.

He was emphatic that scuffing compromises the alclad and is just not necessary, unless the area is already compromised by salts or corrosives (like when you find corrosion under the blue plastic around the edges of alclad that has been stored a long time).

My direct experience doesn't mesh with what your friend told you. I prepped some parts using Alumiprep 33 to etch the oxides away, then Bonderite 1201 to do a conversion coating. I deliberately did not scuff the parts. The object was to see if just removing the oxides would be sufficient for adhesion. I then primed, cured for a couple of days and dimpled. The primer flaked off when I dimpled.

Conversely when I cleaned with Dawn dish soap, scuffed with Prekote and primed, the primer withstood the dimpling process without any damage.

You absolutely need to scuff to get good adhesion. There's no getting around it. I also confirmed this with the Materials & Process people at work (big airplane company that sounds like a spring).
 
Yup..

..buy a new skin and start over. Oops rivets can and will save the day if you have situations in the future that require them. (Ex. you drill out an AN-3 rivet and experience a little wobble that enlarges the hole. Now you need an Oops rivet.) It's not advisable to use them for a large line of problem holes (just get new parts). They're intended as a fix for the very occasional, lone, screw-up.
 
An old-timer I know and respect a whole lot told me that virgin alclad does not need to be scuffed. You're either going to use a conversion coating, and primer. Or, you're going to use a self-etching primer. With clean, new alclad skins and extrusions, just wipe with acetone using a lint-free cloth to remove ink and oils. Then, wipe again, just before priming with denatured alcohol.

My direct experience doesn't mesh with what your friend told you.

My experience is consistent with N804RV advice. When researching whether to scuff or not, I learned that there are 2 methods to "etch". First method is a "mechanical etch", which is simply scuffing with scotchbrite pad or equivalent. Second method is a "chemical etch" which is what "self etching primer" is supposed to do, without having to use mechanical etch. You pick one method, you don't need to do both.

I've used Sherwin Williams P60G self etching, industrial wash primer on my build (Vans recommended) and have been VERY pleased with toughness and results. Have done nothing more than clean thoroughly with acetone, then spray. If I let it cure for 24 hours, it's tough as nails. No flaking off, even when dimpling after primer has cured.
 
Have to add here: that except that small error
we got pretty good priming result:


JUY3AFJr.jpg

Romanov, Looking good!
I didn't look at your tagline to realize you were overseas, so I can totally understand that you were hoping not to order a new skin, but glad to hear you are looking to replace the skin. In the end you'll be happier and have a safer plane. Hopefully the advice here has been helpful and hope to see more of your progress!
 
Romanov, Looking good!
I didn't look at your tagline to realize you were overseas, so I can totally understand that you were hoping not to order a new skin, but glad to hear you are looking to replace the skin. In the end you'll be happier and have a safer plane. Hopefully the advice here has been helpful and hope to see more of your progress!

Yeah, ordered from Vans the new skin yesterday.
It was a good lesson, will keep posting.

BrUzuCh.jpg
 
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