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Paint Blisters/Fuel Tank Rivets

Potentially a VERY good idea!

... they would call in an outside vendor to do a "Helium leak test". Those tiny molecules can pretty much find any opening.
Erik

This is a really good idea, actually. Hellium sniffers are really expensive, but not out of the question for a shop to acquire to do QC on a lot of tanks. The good ones are ball-park $20K. But small hand-held ones for leak checking are about $4K.

So, if Van's really wants to do some quality assurance on QB tanks when they come in, before shipping to customers, they could buy one of the $4K sniffers and leak-check every QB tank. Also, slow-build builders could send their completed tanks in to get tested for a fee.

Now, helium is pretty hard to keep in a tank. It may be that all our tanks would ooze helium. But presumably, it would be a matter of degree, and testing a few would establish a threshold where it is OK vs. one that would leak fuel vapor. It may also be that with some experimentation, a different gas (you can buy sniffers for O2, N2, CO, CO2, variety of others) could be found that would be a more realistic leak rate that would provide a good go/no-go test.

One might argue that it would be more straightforward to train the QB builders to make them better in the first place, but in 3 years, we haven't seen any evidence of that.

Perhaps some vendor testing of some tanks that have been in service and got blisters, and not, would go a ways toward convincing Van's that there is more of a problem that needs addressing? We would need to find a vendor, and arrange for some good samples to test as an experiment.
 
Blisters

Working on my slow build 8 fuel tanks , I was thinking as I was applying sealant to the rivets on the amount of coverage on each rivet . Dan H had some photos showing the " sponge " holes in dried sealant . If rivets are covered with a thin layer would the fuel permeate thru as a gas ( not liquide ) . With that thinking I sealed each rivet with a 5/8 " diam - sealant thick enough not to see any signs of a rivet .
Will report back after a year of full fuel with paint .
 
I'm curious about behavior of Proseal in thicker layers. Most 1-, and 2- part sealants will shrink and tear in thick layers, particularly where the thick material becomes thin, even when it stays adhered. Usually we see this with small mechanical stresses or thermal stresses, both of which could be present in these tanks. Any observations?
 
My plane was Painted in 2004, QB tanks. 2 blisteres appeared last week after 8 years in service. Tempature has been in the 40's. They were next to each other on the botom of the tank. The tank was empty and open to atmosphere for a week before these appeared because I was fixing a small leak on another rivet. I cut off the paint, and there is no sign of fuel.
After fixing my leak, a new leak appeared on the rear flange. My guess is that the QB tanks don't use pro-seal brand sealant, or that they don't abide by the shelf-life dates. This causes the material to not fully cure and degrade when exposed to oxygen. There is then off-gassing that causes the blister. Heat will accelerate the degradation...
 
Paint Bubbles

I don't know if my RV-7a is a QB or not, but I have bubbles.
SN is 71963. Has anyone explored whether the tank caps have not been releasing positive pressure? Don't see a way to attach photosto this message.
 
Bubbles

Here are photos of some examples.

21etd36.jpg


2lcxnxj.jpg


2jcalwk.jpg
 
Welcome to VAF!

Bob, welcome to VAF:D

The fuel caps are sealed, tank venting is done with a dedicated line, should be at the wing root, or on fuse close to the wing root. Look for a bit of 1/4" tube sticking out a short distance, usually has a slash cut on the end.

Good to have you aboard, Sorry about the blisters.
 
Bubbles

Thanks for the reply. Obviously, I didn't build this a/c. I will clean up the bubbles and repaint the areas. Hope the bubbles don't come back.
 
re: Bubbles

For what it is worth I had the same problem on a motorcycle fuel tank. I stripped it, cleaned it real well and re-painted it. The problem seemed to be fixed for about a year and then the bubbles came back in the same spot. In hindsight I should have been more diligent figuring out why they started in the first place. just my 2cents...
 
For what it is worth I had the same problem on a motorcycle fuel tank.

Mike, was the tank welded steel, blown or roto-mold plastic, fiberglass, what? Was the blister on a seam or elsewhere?
 
leaking fuel tanks

Thanks for the reply. Obviously, I didn't build this a/c. I will clean up the bubbles and repaint the areas. Hope the bubbles don't come back.

They will come back in time, you need to fix the source of the leak before going to all the trouble to repaint. I have experienced leaks in factory built tanks on RV-7 and RV-10 tanks. Even a factory replacement tank for the RV-10 leaked. I think there is extremely poor QC on the building and testing of these tanks. The only one I had that did not leak was the ones on an RV-6 that I built.
 
Data point for posterity

Just another data point for posterity:
My RV-6 wings now have blisters.
Data:
1. Slow-build (did it myself), in 1999/2000 time period
2. Std pro-seal methods, no slosh
3. Painted w/Sherwin Williams products (JetGlo/Acryglo)
4. Took 6 years to develop. First flight 2006, first blisters 2012.
5. Correlated with heavier use of 91 octane autogas, laced with ethanol, I started in 2012, although the right wing (which is filled w/100LL 90% of the time) also has the blisters
6. The blisters are still intact, no liquid has ever been noted out of rivets

Plan is to monitor - trying to fix/re-paint would probably make it look worse in the long run, and there is no safety of flight problem at this point.

Cheers,
 
Jordan, where on the tank did the blisters appear, i.e. along the rib line or along the rear baffle, interior rib or exterior rib, etc.? So after six years blisters appeared at the same time on both tanks???
 
Paint blister (semi)repair

My quick-build RV-8 started developing blisters on the tank skins a few months after first flight. It seemed to accelerate when it was out in the sun for a week at Oshkosh. I ended up waiting for about a year for the situation to stabilize before deciding what to do about it.

I figured I would have to repaint the tanks, so I decided to try an experiment. The biggest blister was on the bottom of the left tank, about 1.5 in diameter. I used an Xacto knife to make a small slit, about 1/32 in long in the middle of the bubble. I then used a heat gun (low setting) hoping the blister might shrink down flat. And it did.

This worked so well that I did this on all the blisters. After making the slit (parallel with the slipstream), I would apply some heat for a few seconds and then massage the blister down with a cloth. Then repeated this several times. The result was impressive. Unless someone knows specifically where to look, the repair is not noticeable. It is least noticeable on the white paint, a bit more an the red trim. I did this a year ago and none of the blisters have returned. The result is good enough (so far) that I will not go through the hassle of repainting the tanks

This is, of course, just one data point, and success may be a strong function of the brand of paint used.
 
Welcome to VAF!

Ken, welcome aboard the good ship VAF:D

Good info on the blister repair, another data point for the puzzle, thanks for sharing with us.

Sounds like lancing the blister at first notice may be as good as it gets without having to open the tank??

How about a photo or three of the plane??
 
Blisters

BMW Motorcycles made a painted plastic tank for their GS line around 1994 - 1995 , similar blisters would appear , I had one that was blister free for 2 years until I painted it ( DuPont Chroma Base / Clear ) . First day in the sun it looked like a teenager with bad acne ! BMW warranty removed most from the market and replaced with a metal one .
Were there some microscope photos of Proseal posted ? I wonder if there is a minimum thickness that should be used to cover the shop head of rivets on the inside of the tank ?
Tom
 
Were there some microscope photos of Proseal posted ? I wonder if there is a minimum thickness that should be used to cover the shop head of rivets on the inside of the tank ?

Most of us are using Flamemaster CS3204, while "Pro-Seal" is a trademark of PPG Aerospace. Both are AMS-S-8802 (formerly Mil-S-8802F, Type II) sealants.

Cured sealant is actually much like a sponge, although in theory the voids are closed cell, i.e. not connected. The voids have two sources, (1) air entrained during mixing and (2) bubbles of liquid or vapor toluene, the solvent component used for viscosity adjustment. The solvent evaporates out of the sealant while the chemical cure is proceeding, and apparently for some time after. It would seem the sealant is never totally vapor proof, which is why there is a required thickness.

BTW, there is no test standard for sealant performance with avgas or mogas. All testing is done with what you can think of as a standardized jet fuel equivalent.

Voids.jpg


Thickness over a shop head? 60 mils with a 1/4" fillet.

This is from the original Essex Chemical document "Polysulfide Sealants For Aerospace". They invented the stuff.

Sealant%20Filets%20-%20Essex.jpg


Sealant%20Thickness.jpg
 
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QB fuel tank blisters to :(

My -8A QB has a herd of fuel tank rivet blisters. Not going to bother with them at this time, but wondering if sloshing is the only way (yet) determined to correct the issue?
 
Blisters

Dan , on the quick build tanks and others that you have seen with blisters are they lacking total encapsulation of the shop heads ?
Tom
 
Dan , on the quick build tanks and others that you have seen with blisters are they lacking total encapsulation of the shop heads ?
Tom

Not encapsulating shop heads would violate all the industry standards (ex. SAE AIR4069). It is no doubt a contributor to overall failure rate, but not the problem. Consider blisters along the rear baffle line; shop heads are outside the tank.
 
No silver bullet

.......on the quick build tanks and others that you have seen with blisters are they lacking total encapsulation of the shop heads ?
Tom
Yes, blisters have been observed on quick build tanks that lack shop head encapsulation. It was the dozens of blisters on his quick built fuel tanks that prompted Dave Domeier to start this thread in the first place. One need only review the images in post #100 to confirm that shop head encapsulation was indeed omitted in the construction of those (not Dave's) quick built fuel tanks.

That said, a more in-depth review of this thread will reveal why blistering can occur along the rear baffle and virtually everywhere else for that matter if the fay seal is defective. A proper filet seal is highly recommended and industry accepted practice too.
 
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My quickbuild RV7A has developed 6 separate leaks on rivets of the right tank. This is a 2005 vintage quickbuild. The leaks have all developed within the last year. All have been on the bottom ribs. Looking in the filler neck I can see that the shop heads of the rivets are only encapsulated on one side. It looks like they used an applicator and made one pass down the rib flange, leaving ample sealant on one side of the rivet (the front side in the direction of application) and a large void on the aft side. You can clearly see the shop head and the dimple is clear of any sealant on that side. I presume that was the mode of application used on all the ribs. Close inspection also shows tiny blisters on all bottom rivets of the tank, none so far on the top side. The aircraft has been in service since 2006 and has only been fueled with avgas.

Martin Sutter
Building and flying RV's since 1988
EAA Technical Counselor
 
Response/Add'l data

Jordan, where on the tank did the blisters appear, i.e. along the rib line or along the rear baffle, interior rib or exterior rib, etc.? So after six years blisters appeared at the same time on both tanks???

My blisters are all along the rib lines, in the front half of the tank. None along the rear baffle. Primarily on the bottom, although I have one on the top, near the leading edge. I might try the razor blade/heat gun repair technique on the bottom ones and see how it goes.
 
About to paint a new RV10 and was wondering if someone has the document that Vans released some time ago on this issue. Thanks.
 
Blisters on my -8

QB tanks 2003 vintage.
Professional paint 18 months ago.
One rivet leaked along rear flange. Fixed before paint. No problem
Blisters just showed. 5 on lower left. 1 on lower right.

Is Van's still silent on this issue?
 
Ouch this is not a discussion I wanted to join. I got these $#%^%$# blisters on my wings too, What the Heck??????!!!!!!!!!
Just noticed them. Quick Build 2007
painted one year ago
leak test no problems
Only on the tops, dark blue paint.

Am I looking and pulling of and resealing??????

G
 
My QB wings are of the same vintage, 2006 and both sides, top and bottom blistered like crazy after a year. I was so pissed!! None of the blisters showed any signs of fuel staining, so was strictly internal vapor pressure delaminating the paint. I ended up sanding the paint down to bare metal all along the rib / rivet lines. Then filled the rivet tops with wet micro balloons, laid a ply of fiberglass strip, then dry micro balloons over that. Sanded and filled, no more blisters.
 
Holy a word I cannot use
**** paint job was 10 grand
very disappointing all around. Im this close to flying the darn thing. Guess Im gonna have to add blister repair to the never ending list. Thanks for the info. I hadn't thought about your fix. You did it with the wings on, is that right??
Thx Gary
 
Just trolling around the threads I cam across some one on the 8 sight that has noticed an oil type film around the skins at joints and rivets, apparently they coated the QBs with something for the ocean voyage. I never notice this before, but last week I epoxy primer-ed, wet blocked and left to cure. yesterday I go out to the garage and at the tank seam there was a 1/4" wet line along the seam, look like some type of oil, none of the rivets had this and maybe because I did the Loctite 290 thing. But any way I wiped with grease and wax remover and it did not come back. Painting tomorrow and hope it comes out well. Paint Blisters,Fuel vapors, uncured primer paint thinners, OIL?



Just wanted to do an update and add data on the green Loctite trick, over one year with fuel in the tanks and no problems yet, the paint was mechanical etch with maroon scotch bright pads, epoxy primer, 360 wet sand and then base clear coat, fingers crossed. (above post was from 2012)
 
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I've got about a half-dozen blisters forming on my left wing inboard tank, at 40-ish hours and 4 months after paint. Very small but definitely vapor blisters, all in one cluster near the rear baffle at one internal rib. I'll wait a bit to see if any others pop up before doing anything about it, probably this summer.
 
The last blister survey was circa 2009. It would be interesting to gather fresh data. If you have blisters, and would like to answer a few survey questions, please send a private email.
 
Huh?

I've got about a half-dozen blisters forming on my left wing inboard tank, at 40-ish hours and 4 months after paint. Very small but definitely vapor blisters, all in one cluster near the rear baffle at one internal rib. I'll wait a bit to see if any others pop up before doing anything about it, probably this summer.

Greg, Curious if you had used ?Tank dies? on fuel tanks?
 
Greg, Curious if you had used ?Tank dies? on fuel tanks?

I did, yes, as that was considered the current vogue thing to do at the time I was building my tanks. I used the wet fay-sealing method on all my rivets as well.
 
Holy a word I cannot use
**** paint job was 10 grand
very disappointing all around. Im this close to flying the darn thing. Guess Im gonna have to add blister repair to the never ending list. Thanks for the info. I hadn't thought about your fix. You did it with the wings on, is that right??
Thx Gary

I did it with the wings on. It was a PITA but I?m happy with the outcome.
 
Here is an interesting wrinkle on the blister phenomena - My tanks were built 10 years ago, and I only had two minor weeps that I finally made go away with loctite. The tanks have been unpainted their entire life.

Last spring I wrapped them with vinyl along with the wings. The wings are holding up fine except for the tanks - I'm getting blisters over many rivets. I've cut a few open to find fuel pooled inside. Seems like the vapors are escaping where liquid cannot. I've never used anything but proseal (and a few drops of loctite) on the tanks.

Don
 
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Here is an interesting wrinkle on the blister phenomena - My tanks were built 10 years ago, and I only had two minor weeps that I finally made go away with loctite. The tanks have been unpainted their entire life.

Last spring I wrapped them with vinyl along with the wings. The wings are holding up fine except for the tanks - I'm getting blisters over many rivets. I've cut a few open to find fuel pooled inside. Seems like the vapors are escaping where liquid cannot. I've never used anything but proseal (and a few drops of loctite) on the tanks.

Don

Sorry I can not help with blister solution at the moment, but a question: Would you recommend just applying vinyl wrap ( or a good primer and top coat) to the tanks as a test until the plane is ready for real ($$$) vinyl or paint? Blisters only seem to appear after the big investment.
 
My tanks built in 2013, scotchbrite and MEK wash, can't remember primer, Poly U paint.
Left tank used solely with Avgas,
Right tank used Avgas for 2 years then put in some Mogas. Shortly after it was all mogas I started getting blisters on the top of the wing, above fuel level.
Tried the locktite method with no good result.
Abraded paint and stripped back to bare metal, applied locktite again, then primed with epoxy primer and repainted.
So far there is no more problem, but I have given up using mogas.
What I think happened was that the vapour from the mogas was either so thin that it could get past the proseal, or it somehow chemicaly affected it
 
My tanks built in 2013, scotchbrite and MEK wash, can't remember primer, Poly U paint.
Left tank used solely with Avgas,
Right tank used Avgas for 2 years then put in some Mogas. Shortly after it was all mogas I started getting blisters on the top of the wing, above fuel level.
Tried the locktite method with no good result.
Abraded paint and stripped back to bare metal, applied locktite again, then primed with epoxy primer and repainted.
So far there is no more problem, but I have given up using mogas.
What I think happened was that the vapour from the mogas was either so thin that it could get past the proseal, or it somehow chemicaly affected it


Do they mix alcohol with the gasoline down under? up here 10 percent is normal.
 
I had used Mogas exclusively for the first five years of flying. The last five have been all AV gas.
 
Blisters and Tank venting

I believe that the tank venting design is part of the issue. Why? Simple, when I leave my tank caps open while on the ground in a hot hangar or in the sun, no blisters. Close the caps, blisters! The standard vent design allows fuel to flow into the vent tubes, this creates head when subsequent fuel vapor tries to escape. It's not much, but when I pop my caps after sitting there for a bit on a hot day, woosh, you can hear the pressure relieve itself past the cap as it is opened. This slight pressure surely encourages vapor transmission through the sealant, where it is subsequently trapped by the paint, which in some cases is apparently less permeable than the sealant, and it give way, separating from the skin and forming a blister. As others have noted, once the paint is vented, i use a hypodermic needle, that rivet will no longer blister the paint that has been vented. Soooo, is there another idea for a vent design that does not allow fuel into the vent tube, or at least lessens the potential for back pressure?
 
Vents not clear?

So I am wondering if the vent lines are not clear but rather have some raw fuel causing a liquid plug? This will allow some pressure to build up in the tanks if the internal pressure isnt enough to push the fuel all the way out, or to suck it all into the tank
 
I believe that the tank venting design is part of the issue. Why? Simple, when I leave my tank caps open while on the ground in a hot hangar or in the sun, no blisters. Close the caps, blisters! The standard vent design allows fuel to flow into the vent tubes, this creates head when subsequent fuel vapor tries to escape. It's not much, but when I pop my caps after sitting there for a bit on a hot day, woosh, you can hear the pressure relieve itself past the cap as it is opened. This slight pressure surely encourages vapor transmission through the sealant, where it is subsequently trapped by the paint, which in some cases is apparently less permeable than the sealant, and it give way, separating from the skin and forming a blister. As others have noted, once the paint is vented, i use a hypodermic needle, that rivet will no longer blister the paint that has been vented. Soooo, is there another idea for a vent design that does not allow fuel into the vent tube, or at least lessens the potential for back pressure?

Interesting theory, but when testing the theory we need to consider that the vent design originates all the way back to the RV-4. There are 1000's of RV's with this design, many of them completed in the 80's and 90's with (as far as I am aware) no history of paint blisters.
It has only been within the past 10 years or so that they have been occurring.
My theory is that it is at least in part related to the EPA driven formulation changes that have occurred in paint systems over that time, but that is just a gut feeling.... I don't have any more evidence to back up that theory than any of the others that have been suggested up to this point.
 
I have outboard wing tanks with flow-through venting of the inboard from the outboards, with the outboards vented at the wing tip. The result of that is that I cannot have more than about 2" of fuel column pressure at any time on the tanks - I simply don't have enough vertical pipe runs to get more than that - so my tanks can't pressurize in the manner you describe. I still have gotten some paint blisters - on both inboard and outboard tanks.
 
https://www.dropbox.com/s/3e3khte17hq68ld/IMG_0991.jpeg?dl=0
https://www.dropbox.com/s/15ytz90ramxmz9u/IMG_0992.jpeg?dl=0
Abject despair today... then I found this thread..

2018 QB wings, cleaned with detergent degreaser, wiped down with MEK, then two-pack acid etch primer (P30B) left for 24 hrs then two pack epoxy primer (PR143).
Pressure testing of tanks was fine.
I had 20 litres avgas with fluorescent marker in the tanks for a week with no leaks, then the tanks drained fully and left open for 24 hours, and then the wings stored vertically (leading edge down) with the tanks fully sealed to prevent critters getting in.
Today, I moved the wings and one is fine, the other has 10 blisters, 9 on the lower aft flange and one on a rivet one place closer to the leading edge.
No top coat, and minimal fuel exposure yet Ives still got blisters!
I sanded them all down and obviously no fuel present under them.
Irritating beyond belief, but at least ive found them before spending $10k on a paint job.
So how do I fix the problem? epoxy over the rivets, fibreglass tape and filler?
I feel like setting fire to the bloody thing at the moment! I dont mind sorting out my own ****-ups, but sorting out the result of a "professional' build is maddening!
 
So how do I fix the problem? epoxy over the rivets, fibreglass tape and filler?


I feel like setting fire to the bloody thing at the moment! I dont mind sorting out my own ****-ups, but sorting out the result of a "professional' build is maddening!

How to fix it? Remove the epoxy, clean again, be sure to let the cleaner dry. then re-epoxy. Fiberglass tape isn't going to help.

As far as setting it on fire, most of us have been there. Whether the problem was bubbling paint, the IO-400 engine recall, smoked avionics, or some other unexpected gut punch. Unfortunately, there are likely to be others.
 
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