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Thread: Sealing leaking fiberglass fuel tank

  1. #21

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    I posted some info to this thread in 2011 and didn't revisit until now. I wrote that normal Epoxy Resin will break down from exposure to Ethanol in Mogas, but EAA and others say Vinyl Esther Resin will not. I see that Bob H. has made a comment that this is not true. Bob, if you're still here, please provide your source for saying VE Resin cannot withstand ethanol. You are saying that EAA's info is incorrect.
    As for the recent questions here about sloshing compounds-- there are many and I doubt one can lump them all in together. Some may be tolerant to Ethanol.... But another issue is actual sloshing & coating process. That's dicey. Since you can't see inside the tank as you work the slosh, you don't know if you have gotten that stuff in the fuel exit hole, or the vent hole. Or on the fuel level sensor apparatus, if you weren't able to take it out. The guys who owned my KR-2 before me tried this and failed. Compound was in both the areas I mention. I had to cut the tank open and completely resurface the whole thing.

  2. #22
    George Sychrovsky's Avatar
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    Here is an example of epic sloshing failName:  DSCF0361.jpg
Views: 141
Size:  90.9 KB
    Disclaimer ; opinions of others will vary depending on what they’re selling.

    http://the-grand-design.com/

  3. #23

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    Most polymer laminating systems will eventually breakdown in presence of ethanol and that includes both epoxies and vinyl esters. The resistance to breakdown is in the degree of crosslinking of the resin; higher linking, higher resistance. Most epoxies get the higher linking by heat curing. A 350F curing epoxy will have high linking and will withstand solvents like ethanol. A room temp epoxy has lower cross linking density and will be suseptible to eventual breakdown.
    Polysulphide sealants are formulated to resist breakdown and make good fuel/ethanol seals.
    The Rhino 9700 novalac is a coating with great chemical breakdown resistance, however it is brittle and can develop cracks over time, especially if the tank sees any flexing. I have used the 9700 as a protective barrier for the RT cure epoxy but it eventually cracks and that allows ethanol fuel to come in contact with laminating epoxy. I did find a flexible polysulphide from Spruce ( $38/qt kit) that is brushable and now use it whenever I want to seal tank joints that might flex. Has worked well for 2+ yrs on Pulsar integral glass tanks.

  4. #24

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    Any sort of bond depends on the surface ability to bond. I would think a fuel tank in service is a poor surface to expect bonding. Fuel might contain oils.
    Usually, a good bond requires sanding away or chemical removal of the surface before bond. If the surface had oil or silicone a proper slosh bond may be impossible.

  5. #25

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    At the risk of derailing things, I've always wondered why they haven't come up with a bladder to insert in spaces like that, much like the stuff they make fuel blivets for the Army out of, but thinner.
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  6. #26

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    Cessna 180’s do have rubber fabric bladders stuffed in the wings. They have problems with age.

  7. #27

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    Bob H-- Your comments seem well-grounded in knowledge and experience. I'm sure the tech counselors at EAA who have promoted Vinyl Esther resin as being ethanol resistant would like to have a discussion with you. Seriously, this is critical info that they have apparently gotten wrong and need to correct.

    Bill Berson-- You are absolutely correct about surface prep before applying any resin. I talked to pro fiberglass people about applying to "in service" glass; they said an acetone wash at the very least, sanding if possible.

  8. #28

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    In the bonding business, surface preparation is the most important factor for good adherence. For non-metallic surfaces like epoxy laminates, you would like both mechanical abrasion and chemical activation for the adhesive or coating to bond to. Sanding provides the mechanical "tooth" and solvent wiping just prior to application of adhesive provides chemical activation. This gives a good combination for a strong bonding surface. Actually, a light grit blast is even better.

    On metals, the chemical activation is absent as most metals are not reactive to normal solvents. A variety of metal surface activation processes were developed in the aerospace business like Phosphoric Acid Anodizing (PAA) or FPL etching followed by a heat cured primer. This gives a strong surface for adhesion on aluminum and provides corrosion resistance.
    But for a metal tank that's not open for accessibility, these techniques won't work. A metal tank that can be grit blasted and cleaned would give a good bonding surface.
    If you just throw a solution into a tank and slosh it around, you may not get strong adherence to the metal, and the coating could eventually separate and cause more problems.
    So for a tank made of organic materials (epoxies, vinyl esters, etc), you want to mechanically abrade and chemically wipe at the time of coating application.

    When folks have discussions about dissolution effects of ethanol on tank materials, there are many variables in play like concentration of the ethanol, tank material, temp of operation, time of exposure, and more. Most chemical testing is done on ethanol exposure measured in weeks or months. Yet most aircraft tanks hold ethanol fuel for years. Someone may say that a particular tank material is impervious to ethanol exposure, but you need to make sure the tank material is seeing very long-term exposure before giving it a blessing. My technical opinion is that most homebuilt structural laminating resin systems will be susceptible to ethanol exposure if the surface is left bare and you need a coating to keep the ethanol from the resin to maintain laminate integrity.

    Sorry for the long answer.

  9. #29

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    Quote Originally Posted by Bob H View Post
    Most polymer laminating systems will eventually breakdown in presence of ethanol and that includes both epoxies and vinyl esters. The resistance to breakdown is in the degree of crosslinking of the resin; higher linking, higher resistance. Most epoxies get the higher linking by heat curing. A 350F curing epoxy will have high linking and will withstand solvents like ethanol. A room temp epoxy has lower cross linking density and will be suseptible to eventual breakdown.
    Polysulphide sealants are formulated to resist breakdown and make good fuel/ethanol seals.
    The Rhino 9700 novalac is a coating with great chemical breakdown resistance, however it is brittle and can develop cracks over time, especially if the tank sees any flexing. I have used the 9700 as a protective barrier for the RT cure epoxy but it eventually cracks and that allows ethanol fuel to come in contact with laminating epoxy. I did find a flexible polysulphide from Spruce ( $38/qt kit) that is brushable and now use it whenever I want to seal tank joints that might flex. Has worked well for 2+ yrs on Pulsar integral glass tanks.

    Are you refering to the Bill Hirsch automotive gas tank sealer ( p/n 09-03477)? The description did not mention polysulphide.

    Does anyone else have experience with this product?

    Wayne

  10. #30
    cub builder's Avatar
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    Quote Originally Posted by Eckertwa View Post
    Are you refering to the Bill Hirsch automotive gas tank sealer ( p/n 09-03477)? The description did not mention polysulphide.

    Does anyone else have experience with this product?

    Wayne
    I sloshed the Epoxy resin glass tanks in one of my planes when I was building it using the Bill Hirsch Alcohol resistant sealer. The tanks have been in service for 22 years now, and well over 1200 flight hours with no hint of delamination or peeling. I did slosh them when they were new, so had never had any oils or petroleum products in the tanks. Sloshing tanks requires proper prep for proper adhesion, just like painting.

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