This one in particular had to be repaired in place, as the hole in the cabin sole isn’t big enough for it to pass through, and I’d sooner cut the tank into little pieces than cut out a bamboo w/ teak battens cabin sole.
An explosion, though initially worse; would have had essentially the same results. The inside of the holding tank turns everything outside the holding tank just like the inside. Shitty. The entire bilge was awash in my friends’ poop. Their so called clean diets and clean living did fuck all for the chemistry of the bilge water. The night that Olivia and I moved aboard, we slept in a holding tank simulation. My top priority as this “big white boat’s” new Captain was etched into my olfactory nerves. So once I’d cut the top off the tank, the area of concern made it’s self apparent. 
The first layup of 8oz cloth tape strips, cut to fit the corners, was wrapped around a couple of reinforment battens that I cut from a piece of 1” thick PVC board, and shaped to fit snugly against the separated tank walls that needed to be rejoined at the corner.
The first layup was just to seat the battens. About half a pound of West Systems fast cure epoxy was used in this step. It’s probably water tight. But we aren’t holding just water ...and the tank is going to endure interesting stressors. My intent is to make certain that the tank it’s self, never again becomes a “stressor”. 
After a fastidious grind & sand session, the now steady walls could handle a more aggressive layup. Probably a full 16oz or more epoxy was used today. Still locking it down with the tight weave 8oz cloth, this time in at least six directions around the cracks and corners. The key to adhesion is prep. Gouge away all that shimmers with sanders and die grinders. Then wipe it clean with acetone as if you feared that the captain might surprise you with an inspect-it-with-your-testicle adhesion preparedness test.
We’ll repeat those steps with varying materials and techniques about 8 more times. For me to poop on!
Although my upvote is small I am happy to share it with you.
Hi, I like your work. I understand the seriousness of this incident. Instead, I wish you to dip into a glass of champagne more often
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