I have a shitty old hot tub in my backyard. It came with the house, but it wasn't well taken care of and the internal plumbing is cracked, so I was thinking this summer of turning it into a fish pond. The inside is a light gray (ugly) and I don't know what chemicals may have been used, so I was thinking of coating it with spray on truck bed liner. Has anyone done something like this before? Any thoughts?