I was going to recommend por 15 too,while it is a good product after reading all the details I might be looking into something else if it was me.The reason being I surely wouldnt want to be crawling in a dark 40 ft deep culvert again for a real long time.I think there has to be a 2 part epoxy paint for this job,I just have no clue as to what it is.I'd be looking at stuff aimed towards ships and bridges if it was me.
I just sent a email to a friend in the commercial fishing industry,if he is in port he might have a recommendation.
