I found a really cool DIY koi pond liner and coating comparison

https://store.sprayliningandcoatings.com/comparisons/best-diy-professional-grade-koi-pond-coatings-for-2018/

I
learned specific differences between the DIY stuff versus
"professional" grade polyurea koi pond liners. This SLC report thing
educated me about differences in epoxy vs polyurethane vs polyurea… It’s
like a 2nd business after finishing 4 koi ponds. I’ve done a
lot of research on Spray on koi pond liners, the hard ones vs the
“correct” coatings. I’m a full time landscaper so its not my main
business but this DIY koi pond liner made of polyurea held up at 1
client for 6 years now. Others were under 2 years, however.

http://spray-lining.net/seal-tite-pond-coating-product-information