"fiberglass gas tank"? What cars would have a fiberglass gas tank?
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The husband wasn't warm to the idea of putting a coating inside the tank although I don't recall why.
The fibreglass gas tank was on one of the Eagle parts-cars I got. It was a wagon too. Aftermarket replacement. Perfect fit to the stock size, straps, fill pipe & gasket, etc.. Seller made sure I knew about it so I'd grab it before scrapping the vehicle once I'd removed what I wanted. Needed a ground wire to the fuel level sender. I moved it to my '81 with its 119. I'm sure I kept that tank when the '81 went to scrap, but I don't recall where I stashed it. Think it's in my storage locker.
You don't want to guess when you put something inside the tank, as that may dissolve and go to the engine as gunk, or in solution and precipitate in the carb, etc.. You want something that is intended for, and rated for,
gasoline, ethanol and steel tanks. Due to the ethanol in fuel, and the can's & sealer's age, I wouldn't use the old can of sealer I have from the aviation friend back in the 90's. Consider modern fuel additives too. Verifying it's good for your intended usage is NOT done by the online seller saying "sure it is!" Who makes it, what's the installation prep & procedure, and what's the warranty. Google turns some up, but you'd have to do your research. I'd want to find out what was in the liner, so I could look for products that failed that used those.
As to
if you should line it after you de-rust it, look to the vehicle restoration enthusiasts, where they're dealing with various degrees of rust, and possibly cracking, etc.. Simply pulling it and doing the de-rusting may be enough. If it hasn't lost much thickness to rust inside (I've no idea how to gauge that), I'd consider brushing or rolling some epoxy resin on the outside for rust proofing (may or may not be cheaper than DIY epoxy primer, or having a shop give it a flash coat of epoxy primer the next time they're shooting that), then a top coat of something like Waxoyl Hardwax.