What do you think gas station fuel storage tanks -- the ones underground -- are made of? Well, I wondered too, so I Googled it. Here's a prominent result:
https://www.nwpump.com/petroleum/petroleum-products/storage-tanks/
They sell tanks. They say that steel tanks are cheapest. Then they go on to say, and I quote:
Fiberglass tanks are a rust-proof option in this age of ethanol fuels and biodiesel fuel products, offering some distinct advantages to traditional steel.
So, believe what you want about fiberglass motorcycle tanks.
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I built the above bike in 2014. That's early Omar's glass. The tank was already several years old when I got it in 2014. I don't remember if there were any signs of it ever having gas in it when I got it. I used to have a pic of Kenny Roberts on an XS650 that had the same tank with the exact same graphics on it -- which I'm pretty sure were paint, not decals, if that's possible.
Anyways, at that time, 2014, Omar's said that the only tank liner that worked with fiberglass was Caswell. They would Caswell the tank that you were buying from them for a little extra. Yes, ethanol gas was everywhere in 2014.
So, I lined the tank with Caswell's. Used nothing but E10 in it. Never drained it. Rode it for a least year after I completed it. Sold it to a guy in south Texas. He'd call me every couple years to tell me how much he loved it. Never had any problems with the tank.
I've used Caswell many, many times since. It now comes in several colors, black, blue, grey, red, etc.
I've lined over failed Kreme liners more than once with no problems.
The only time I ever had a problem was when I made the mistake of measuring the proportions by weight instead of volume. It would not harden, but turned rubbery. I fixed it by re-lining with properly proportioned Caswell's. That was several years ago and the tank has been on my daily rider ever since.
By the way, I asked Caswell to give me the by-weight mixing proportions, and they didn't know what they were, a real shortcoming if you ask me. I mean, how hard can it be for them to figure it out? I have since calculated the by-weight proportions myself, not hard to do.
Final word: yes, E10 is nasty, but it's really nasty when it gets old. You need to keep cycling it, in other words you need to ride your bike enough that you're putting fresh gas in it like every three months or so at the most. If the gas is going to sit longer than that, I strongly recommend an ethanol fuel stabilizer. Which is a topic for a whole 'nother thread, and there are several on this Forum. The stabilizer I think is best is called "Ethanol Shield." It was little-known in 2014, but now Lowe's, Home Depot, and Walmart have it in-store.
Final final word -- I ride my bikes enough that I see no need for Ethanol Shield. But I ALWAYS used it in my Caswell's-lined Omar's tank, and I told the guy I sold it to to do the same.