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Overcoating sealer outside the tank

170 driver

Well Known Member
I was wondering if anyone has found anything that can be used to overcoat the exposed sealer on the outside and rear of the tank to prevent the blue dye from a small leak from softening the sealer in a big area and causing massive rework? It seems that there should be some coating that is impervious to the blue dye. This could make small leaks an easier repair.
 
It's not the 100LL that softens it, otherwise the sealer would be soft inside the tank. It is only when the fuel evaporates and leaves the blue dye on the sealer that it becomes soft. It must have an effect on the hardener that is in the sealer. If you have an rv, you will eventually see this I'm about sure. I don't think it is possible to construct a completely leakproof tank, my right tank held for 9 years till a sending unit screw just started seeping onto the sealer and softened it. The left tank only lasted 2 years before I had to repair it.
 
I'll go one step further - It's probably neither the fuel nor the blue dye. If the sealer is softening, it's more likely that it wasn't fully mixed, or wasn't mixed in the right proportions.
 
I'll go one step further - It's probably neither the fuel nor the blue dye. If the sealer is softening, it's more likely that it wasn't fully mixed, or wasn't mixed in the right proportions.

What he said.

E

Search the archives here in the forums... there is a lot of past discussion.
One in particular..... My cured Pro-Seal has turned to goo!
http://www.vansairforce.com/community/showthread.php?t=122300&highlight=pro-seal

It will indeed happen with sealant that was properly ratio'ed and mixed.

How do I know?
I have seen it happen on sealant that had been (thoroughly) mixed from one of the small pre-portioned kits (besides numerous other times).

To answer the original question - If all of the softened sealant can be removed and the surface well cleaned, fresh sealant will adhere very well to old/cured sealant. This will sometimes resolve a leak depending on what the leak source is.
 
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To borrow from Hans and Franz: "Hear me now and believe me later!"

The only way to accomplish what you are trying to figure out is to figure out a different way to build an RV fuel tank. Be assured there is only ONE way to stop the blue dye (fuel) from showing up on the outside of a fuel tank seam. Find the leak, open the tank up and reapply the proseal from the inside of the tank. No amount of attempting to seal from the outside will affect the leak.

Ok, go ahead. Ask me how I know!
 
To borrow from Hans and Franz: "Hear me now and believe me later!"

The only way to accomplish what you are trying to figure out is to figure out a different way to build an RV fuel tank. Be assured there is only ONE way to stop the blue dye (fuel) from showing up on the outside of a fuel tank seam. Find the leak, open the tank up and reapply the proseal from the inside of the tank. No amount of attempting to seal from the outside will affect the leak.

Ok, go ahead. Ask me how I know!

I agree that you are correct for the majority of leak instances but tiny leaks do occasionally get repaired by sealing from the outside which is why I wrote "depending on what the leak source is".
 
I agree that you are correct for the majority of leak instances but tiny leaks do occasionally get repaired by sealing from the outside which is why I wrote "depending on what the leak source is".
I speak from my personal experience (7 of them actually). I did have experience with very very small pinhole leaks. In multiple attempts to seal those small pinhole leaks from the outside not one of the attempts worked for more than a couple of months before they started leaking again. I am now coming up on 4 years of leak free flying after drilling out 5 four inch diameter holes in each bay of my left fuel tank, resealing with proseal all seams on the inside of the tank, and sealing up the holes with the Vans Fuel Tank Repair Kit (5 of them).

It is somewhat concerning to think about removing a fuel tank, opening it up, and especially drilling a great number of new holes into the tank (including one great big hole). Given that, I will say with confident assurance, doing so does work.
 
I can say with certainty that the Proseal 890 sealer was properly mixed and in the correct ratio as weighed on a digital scale and well within expiration date. Only the areas with blue stains are soft and the rear baffle was sealed with all the same mixed batch. My left tank has the 4 holes cut in it with the Van's repair kits, it continues to hold after a year. It just seems unnecessary for a 1 foot section of the rear baffle lower seam and the inboard lower rib to all be soft and have to be repaired due to a tiny leak at one fuel sender screw. I know if I don't repair it properly from the inside it will eventually leak in those areas due to soft sealer. I have talked to a knowledgeable guy on polysulfide sealer and epoxy coatings and he said the chemical that is breaking down the sealer is breaking it down at a molecular level and not just deactivating the hardener in the sealer. I will brush on two part epoxy Scotchweld DP460 on all the exposed sealer outside the tank, he says hardened epoxy is the most chemical resistant coating he can think of. It can't hurt. Fuel leaks are the pits.
 
Search the archives here in the forums... there is a lot of past discussion.
One in particular..... My cured Pro-Seal has turned to goo!
http://www.vansairforce.com/community/showthread.php?t=122300&highlight=pro-seal

It will indeed happen with sealant that was properly ratio'ed and mixed.

How do I know?
I have seen it happen on sealant that had been (thoroughly) mixed from one of the small pre-portioned kits (besides numerous other times).

Huh. Do you have any explanation for what exactly causes this, or a solution for stopping it from occurring? Its a little disturbing to think that the very substance recommended by Vans to seal tanks is, in at least some cases, turned to goo by ingredients in aviation fuel.

erich
 
Huh. Do you have any explanation for what exactly causes this, or a solution for stopping it from occurring? Its a little disturbing to think that the very substance recommended by Vans to seal tanks is, in at least some cases, turned to goo by ingredients in aviation fuel.

erich

It only turns to goo if you have a leak and ignore repairing it.

Instances of large areas of softened sealant are instances that the leak went undetected for quite a while (my experience anyway).

The very substance recommended is the only substance that will work (or proseal brand but it will do the same thing if a leak goes undetected or ignored).

It is cause by long term exposure to evaporating fuel.
The way to prevent it is to make sure the only path that fuel is exiting your tank through is the fuel pick-up.
 
You are exactly right Scott. It is impossible to inspect the entire rear baffle area or the outboard rib with the tank installed, so once a leak is detected, it is most likely a long term seep. I just thought if I could protect the sealer by overcoating it on the outside of the tank to isolate it from exposure to the evaporating fuel that it would either limit the amount of damage to the sealer in areas that were not previously leaking or prevent the softening all together. It will be an experiment and I hope to have no more leaks so I will never find out if it works or not.

Note to others who may be interested: If you see blue fuel stains anywhere on your tank or wing, investigate and correct immediately or the repair becomes a much larger task.
 
It's not the 100LL that softens it, otherwise the sealer would be soft inside the tank. It is only when the fuel evaporates and leaves the blue dye on the sealer that it becomes soft. It must have an effect on the hardener that is in the sealer. If you have an rv, you will eventually see this I'm about sure. I don't think it is possible to construct a completely leakproof tank, my right tank held for 9 years till a sending unit screw just started seeping onto the sealer and softened it. The left tank only lasted 2 years before I had to repair it.

This doesn't make sense. If its the blue dye in 100LL that's causing the sealant to soften, then the sealant in NOT impervious to 100LL. I would think there is lots of blue staining inside the tank near the upper outboard sections of the tank where there's lots of evaporated fuel. And yet, fuel leaks are often from the inboard rear baffle if I'm not mistaken. This is an area that is in contact with liquid fuel for much longer periods than the upper outboard sections.

Should the blue dye be "rinsed" out periodically with auto fuel or something else with no blue dye? I doubt it and feel something else must be going on here.

Bevan
 
no, when you open the tank there is no blue dye staining anywhere, it only happens when it evaporates outside the tank. I guess there is not enough oxygen in the tank or airflow to cause the fuel to evaporate inside. Sort of strange, I tried to get a msds sheet on the blue dye and it doesn't seem to exist. Apparently different gas companies use different mixtures of the dye. Heck, it may not be the dye that is doing it but residue from some other chemical in the 100LL fuel evaporation residue, but something in the residue is causing it. I'm not sure if anyone who uses straight mogas has the problems or not. I have never seen any reports on that. Just Avgas and the blue stained areas that are soft. The blue dye is a solvent dye, I think it is either solvent blue 26 or solvent blue 35. It is not a pigment in the sense of the word.
 
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This doesn't make sense. If its the blue dye in 100LL that's causing the sealant to soften, then the sealant in NOT impervious to 100LL. I would think there is lots of blue staining inside the tank near the upper outboard sections of the tank where there's lots of evaporated fuel. And yet, fuel leaks are often from the inboard rear baffle if I'm not mistaken. This is an area that is in contact with liquid fuel for much longer periods than the upper outboard sections.

Should the blue dye be "rinsed" out periodically with auto fuel or something else with no blue dye? I doubt it and feel something else must be going on here.

Bevan

I don't think evaporation happens with fuel in a closed fuel tank any more than evaporation happens in a sealed water bottle.

Once the air space in a water bottle reaches 100% humidity no more water evaporates.
I imagine fuel does the same but I am not smart enough to know for sure.

I have never seen fuel stains inside an aircraft fuel tank.

Keep in mind the percentage of dye in avgas is pretty low. It takes quite a bit of evaporation time for it to start building up. That is why I said you should not ignore a leak. If you find it because of a dye stain, it has probably already been leaking for a while.
 
Don't know that it's relavent to the discussion, but...

Aircraft gas tanks aren't sealed. Evaporation % can be significant. When we kept an avgas tank active on our private field, 10% shrinkage while going through a thousand gallons (over several months) wasn't uncommon.
 
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