POR15 is heavily criticized by people who use it. Whether that's from lack of prep or product failure I have no clue. I used Redkote on my 38 years ago when replacement tanks were not available. It lasted 15 years before I started noticing issues. The problem with coating is it's almost impossible to get the whole surface coated and anything not coated continues to deteriorate. There are outfits like Gas Tank Renu that cut holes in the tank to get everything prepped and coated then close the holes up. They offer a lifetime warranty but are very expensive. I would recommend buying a new tank if they are available.

I used POR15 on my gas tank.  The tank was full of sludge and rust.  I could not find a shop that would clean it for me, so I used the chain, screws, nut and bolts and small gravel shake, rattle and roll cleaning method.  After it flushed clean, I rinsed it out with some muriatic acid, rinsed it out with water then flushed it out with a baking soda water rinse mixture and then rinsed it out with soap and water.  Made sure it was dry and followed the instructions on the POR15 can.  I can't comment on the longevity of the product, but it did cure nice and hard as promised.  It did take some time to get everything clean but all I can say is to clean, clean, clean, and it will give you the best results.

A friend's POR15 experience. A result experienced by many. https://www.facebook.com/100004023956757/videos/1458613615031423

Posted by: @greg-stark

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Has anyone used POR 15 to coat the inside of your gas tank? I've got some rust and dirt in my 1932 Roadster 

A while ago, I looked into POR-15 and found that some folks love it and some not so much.

When I needed to coat the inside of my 1928's vacuum fuel tank, I used just the Metal Ready liquid from POR-15, not the POR-15 coating itself. The Metal Ready leaves a zinc phosphate coating on the inside of the tank. For cleaning the tank out and prepping for the Metal Ready, I used a handful of stainless steel nuts and bolts, and some DAWN dish soap.

The details of the vacuum tank rehab (along with a carb rebuild), are here: https://vccachat.org/ubbthreads.php/topics/458708/fuel-problems-investigation-and-fixes.html

Cheers, Dean