Corrosion under plastic

RVbySDI said:

That picture brings up a question. I cannot think why one would drill and countersink holes with the blue plastic still attached to the piece. That plastic came off as soon as possible for me. Once you find your first sign of corrosion underneath some of that plastic you will understand why.

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I didn't want to hijack the thread about countersinking that this came from. I've seen this topic come up from time to time and I wanted to comment. 23 years building on two projects and I've never seen corrosion under the plastic. Not to say it doesn't happen but... the plastic is over aluminum cladding, so why would it corrode? Something like dissimilar metals or a reaction to the adhesive? I'm guessing what people are seeing as surface corrosion is actually the adhesive getting old but, again, I could be wrong.

However, to answer why you'd want to drill and countersink with the plastic on (or dimple, etc.) is to preserve the cladding. The plastic helps prevent the ribs from scoring inside the leading edges of the skins, for instance, during assembly. If you are going to prime, no big deal, but even then I didn't see any reason to risk gouging the cladding if I can avoid it.

I worried that maybe the thickness of the plastic would cause a fit problem but the only place I have found that to be true were the spacers for my fuselage spar parts on the RV-10; it took that many layers of plastic to make a difference. But since the finish on the spacers didn't matter, I stripped them no problem. For the rest of the aircraft, one or two layers of plastic has not affected the fit at the final step when it was removed.

The other problem people have had is removing the stuff after it gets old. I've not had any problem with the blue or clear plastic (haven't seen clear since the RV-6A wings, around 1997). My -10 kit arrived in 2009 and I'm still having no problem; all it takes is some heat and the parts are dust and scratch free after having been protected by the plastic. I've seen the white plastic on someone else's project and it truly was a PITA; not sure what I'd advise there.

I mentioned dimpling with the plastic and I've had two issues there, both easily solved. First is that some parts with gather chips under the plastic from drilling and you definitely don't want to dimple over those chips. When this happens, the chips are between the surfaces so the underlying rib gets the plastic stripped before using a squeezer to dimple; I can easily keep the dies from scratching the cladding here. For the skin, I strip the inner surface to remove the chips but leave the outer plastic on to keep the male die from scratching as I move the skin in the C-frame tool. Removing the inner plastic also prevents the dies from cutting the plastic and leaving little dimple-sized circles of plastic on the skin when the plastic is removed. I've never seen this cutting happen on the male die side.

I'm not advocating leaving the plastic on, though it's what I do. I suspect this is like the primer wars; everyone has an opinion and yet whatever path is followed the plane will still fly. I just wanted to answer the implied question posed in the quoted post.

In dry climates, we don't need to worry much about corrosion. Don't even need primer, for that matter.

Some of the parts for my RV-3B had tags showing their manufacturing dates and some were pretty old. Some of these had corrosion underneath. The corrosion was light and easily removed, not to worry, and the interesting thing is that it only occurred where the blue film had lifted up in an area. Where the blue film was secure, no corrosion. There were a number of areas like that. The film tends to lift in small elongated bubbles near each other as if the aluminum had been rolled up at some point.

It wasn't difficult to use a soldering iron to remove the blue film at holes or in areas where parts join. I think removing it in those areas is good practice because it avoids any issue of dimension (the film seems to be about .0015 to .002 inches thick) or fit, and with the film gone, it's easier to deburr or see how the joint is developing during fabrication. Also, the film tends to lift at any edge and moving the edges away from the holes avoids problems of the film bunching up or wrinkling in awkward locations, as well as trapping chips.

Dave

Come down to the southeast. Seen this more times than I'd like - sometimes only a couple months after receiving from Van's:

flion said:

I didn't want to hijack the thread about countersinking that this came from. I've seen this topic come up from time to time and I wanted to comment. 23 years building on two projects and I've never seen corrosion under the plastic. . .

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One very important point that you should clarify is WHERE you have built two airplanes in the past 23 years. I note you are in Flagstaff, AZ. A very arid climate. If indeed you have built in that climate then I can see very easily why you hold to your belief. However, builders who are not so fortunate to be able to have such an arid climate beware. Introduce any moisture between that plastic and the aluminum and you will see very similar evidence as is posted in the pictures above.

This may be just a little off topic but when I think of things such as corrosion, rust, erosion, deterioration or any other such destruction of man made items I cannot help but think about the nature of existence on this planet. Man wishes his inventions to be everlasting. Nature wishes her inventions to last as long as immediately needed then transforms them into something else that is needed. Corrosion, rust, erosion, deterioration are just mechanisms for nature to recycle. The physics of nature is such that as soon as something is created there are other forces set in motion to break it down into something else. Change is inevitable no matter how much man wishes it were not.

for those who believe plastic is good, let me suggest an experiment. Take a piece of untreated plain steel. ligtly spray water on one side and on the other place a wet towel. Come back in a few days and observe the results. It is my belief that taking steps to retain moisture in contact with anything that can corrode is a bad idea. My kit sat for 10 years in the midwest. Several alclad pieces has corrosion on the edges where it can trap moisture and hold it in contact. Corrossion is due to moisture wicking in between the plastic and aluminum and not easily dissipating. The open edge allows a path in.

Larry

PIREP from the Northeast.




Plastic removed.

Opinions vary, but having been RVing since 1987, my opinion of the plastic film is that it comes from Satan and will lead to the downfall of mankind.

Leave that stuff on a part, store it in the average hot, humid mid-western garage and it will likely look much worse than Jon's photo. And if you're really lucky, it will peel off in tiny strips too.

Having said all of that, the stuff that is stored inside my air conditioned shop is no problem, but I peel it off any way. Who needs the risk of problems?

Well, I guess if you're actually going to polish your plane, it might be beneficial, but, as we all know, only crazy people want a polished aluminum aircraft.

But seriously, if Van's offered parts without that stuff, I'd pay extra so as not to have to waste time carefully peeling all of it off. Gack!

Ever try to get an old layer of film off of the .016" thick control skins? Grrrrrrrrrrr.

60av8tor said:

Come down to the southeast. Seen this more times than I'd like - sometimes only a couple months after receiving from Van's:

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vfrazier said:

Leave that stuff on a part, store it in the average hot, humid mid-western garage and it will likely look much worse than Jon's photo. And if you're really lucky, it will peel off in tiny strips too.

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Which is why in Paragraph 5.12 in Section 5 of the construction manual it says....

The adhesive on the vinyl strengthens with age, so if the coating is left on for more than a few weeks, it may become very difficult to
remove. Corrosion has been found under the vinyl in some instances. If vinyl covered parts must be stored for long periods remove
the vinyl first.

People don't start their build without reading the manual do they....

White Plastic

was incredibly difficult to remove on my early rv8. I tried everything to no avail, until someone suggested... paint remover. It is messy, a royal PITA, but works. Have had no issues getting the blue stuff off, even after all of these years in the Pacific NW.
woody

RVbySDI said:

One very important point that you should clarify is WHERE you have built two airplanes in the past 23 years. I note you are in Flagstaff, AZ. A very arid climate. If indeed you have built in that climate then I can see very easily why you hold to your belief. However, builders who are not so fortunate to be able to have such an arid climate beware. Introduce any moisture between that plastic and the aluminum and you will see very similar evidence as is posted in the pictures above.

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Good point. 4 years in Ann Arbor, MI and 4 years in Chicago. And Flagstaff is NOT arid; you are thinking of Phoenix. We have forests and snow up here. My hangar is prone to ice on the floor in winter, and floods in monsoon season.

And I've seen deposits like that on my alclad before; it's like bathroom scale. Very annoying to remove, but the aluminum underneath is fine. Real corrosion does not generally look like a feathery crystalline deposit; it looks bubbly, sometimes like worm tracks. Not to say don't worry about it; when it gets as bad as the picture Vlad shared, it can lead to real underlying corrosion.

Anyway, everyone's mileage varies. Maybe I've been lucky but I've never taken particular care to work only in a dry place. In the Spring I have to place strategic covers over my project because the condensation from the hangar roof drips all over everything; I don't bother to cover the parts storage. When the -10 finally flies, people can judge for themselves because aside from exterior paint and non-clad parts, nothing is getting primed. So if there's anything other than shiny, it'll be apparent.

vfrazier said:

Opinions vary, but having been RVing since 1987, my opinion of the plastic film is that it comes from Satan and will lead to the downfall of mankind.

.......
But seriously, if Van's offered parts without that stuff, I'd pay extra so as not to have to waste time carefully peeling all of it off. Gack!
......

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Vans used to ship the aluminum parts with no blue film in the early kits. They got stacked in the shipping boxes with thin sheets of packing paper as separators to prevent scratches.

If builders left their boxes opened but still packed on cold garage floors the papers soon absorbed moisture and corroded the sheets. It was probably worse than the blue film example above.

Like Gil said....er, implied.... get to building and all of this is moot!!!