Paint blister proposal for Vans resolution

[az_gila]

Originally Posted by DanH

Gil, I'm still not following. You're saying a thick layer of proseal is bad, or that this particular design lacks fasteners, and the designer expected proseal to bridge large gaps that flex and move?

Both...

But really pointing out that the Grumman common leak spot has nothing to do with possible surface paint blisters since there are almost no rivets in the outer skin.

The common leak is at the top or bottom of the spar at the tank root rib, or around the fuel sender, which is almost identical to the RV setup.

The tank design dates back to around 1974.

[mspenc45]

Got it, Grumman not a good comparison. How about Mooney? From what I heard from Evan's, Mooney's are a real mess.

[DanH]

I think you have to separate "leaks" from "blisters" in all cases. Survey evidence says wet blisters are rare. And thickness of a filet seal is unlikely to contribute to leak formation, unless the problem is lack of thickness.

FWIW, the lab was able to detect lead in a wet blister, confirming fuel rather than some other liquid source. They were not able to isolate lead in a dry blister sample. However, that fact is a bit inconclusive, as the vapor pressure of TEL is very low (0.2 mmHg). It would be difficult to transport much lead in vapor phase through sealant.

[David-aviator]

Originally Posted by DanH

I think you have to separate "leaks" from "blisters" in all cases. Survey evidence says wet blisters are rare. And thickness of a filet seal is unlikely to contribute to leak formation, unless the problem is lack of thickness.

FWIW, the lab was able to detect lead in a wet blister, confirming fuel rather than some other liquid source. They were not able to isolate lead in a dry blister sample. However, that fact is a bit inconclusive, as the vapor pressure of TEL is very low (0.2 mmHg). It would be difficult to transport much lead in vapor phase through sealant.

If in fact a pressure build in the tank is causing the blisters, relieving that pressure by loosening the fuel caps when parked would prevent it.

There have been reports of tank pressure build up due to some anomoly with the some vent systems. I do not understand how that could happened but it has been reported.

Perhaps the explanation is as simple as that.

[aerhed]

Q: How many have used the Flamemaster, and how many have used 3M or DeSoto? Which goo does the factory use?

[mmartin1872]

Any chances the issue is not the pro seal. . But the type of pro seal and the method of mixing.. when I was building the cj3 center wing box we would use premixed and frozen prc for the fuel tank fillets and laps.. it came from our supplier "spun" to remove the air out of the prc.. because as you mix it you beat a little bit of air into it just like making batter.. this air would be microscopic but it would be there. .


Maybe the secret is going to be how you mix your prc, which determines if you are going to get blisters. Because obviously in the homebuilt world you arent going to be able to buy the pre mixed and frozen pro seal.

If I remember correctly our pro seal also had micro balloons in it to add volume and reduce weight. . But don't quote me on that.

Mike

[DanH]

Originally Posted by mmartin1872

it came from our supplier "spun" to remove the air out of the prc.. because as you mix it you beat a little bit of air into it just like making batter.. this air would be microscopic but it would be there.

Entirely true. However, the manufacturer's technical rep told me it's not possible to create a true solid, just smaller voids. Even the toluene solvent leaves voids when it evaporates...just turn up the magnification. Cured proseal is a closed cell sponge.

BTW, toluene evaporates out of the sealant for a few weeks, which tells us the stuff has a vapor transmission rate.

Taken at 10X:

Originally Posted by aerhed

Which goo does the factory use?

Flamemaster CS3204B2

[Doug]

In applications where voids (like these) are not permitted common practice (where I worked) was to briefly subject the mixed batch to a very strong vacuum. The trapped air will 'boil off' over about 30 seconds. This was very effective at removing these voids but care needed to be taken if the products contained lower vp components. (The vacuum pot I used was purpose built with a 3/4" plexi lid.)
My experience with this process was with polyurethanes only, I never personally tested polysulphides, although I did give it some thought while building my tanks.
With the toluene reduction - I believe the addition of toluene is the only difference between the viscous and much less viscous product variations. Given this I once tested some samples with up to about 50% added to the Flamemaster mix and found a softer final product and significantly longer cure time. I did not test for changes to the properties beyond what I was specifically interested in at the time but I got the impression the toluene mixed at a molecular level so should not have introduced voids. Remember the outgassing is (or should be) diffusion at a molecular level.

[RVG8tor]

I have QB tanks and on top of one wing is a blister that has enlarged to the point it is bigger in diameter than the rivet under it. I wonder if it is time to pop the blister.

I personally think it has more to do with vapor. I live in a nice cool climate and this blister was hardly noticeable until I flew to OSH the first year I had it, it was warm there, the blister really bulged up. They plane has been flying for nearly 3 years now so the growth is very slow. I did not have any others till once when the plane sat with 1/3 tanks, now along the top and bottom at the rear spar there are very tiny blisters starting to form in a hand full of places.

Cheers

[johngoodman]

Originally Posted by RVG8tor

I have QB tanks and on top of one wing is a blister that has enlarged to the point it is bigger in diameter than the rivet under it. I wonder if it is time to pop the blister.

I personally think it has more to do with vapor. I live in a nice cool climate and this blister was hardly noticeable until I flew to OSH the first year I had it, it was warm there, the blister really bulged up. They plane has been flying for nearly 3 years now so the growth is very slow. I did not have any others till once when the plane sat with 1/3 tanks, now along the top and bottom at the rear spar there are very tiny blisters starting to form in a hand full of places.

Cheers

I've had the blisters on the QB tanks since it was painted 4 years ago. I used to get a lot, but now it's settling down. I keep an Exacto knife with a tiny #1 blade to lance the back side of the blister - it works most of the time, but in some cases, the blister eventually blows off anyway. In my opinion, the blisters are more frequent with warmer temps.

John