These thanks are filthy, bought the boat a few years ago and they were filthy then. I tired cleaning
them with bleach, vinegar, water, but the growth just came back.
The tanks don't leak, just the 4" beckson ports. The tanks are fiberglass (likely polyester, since they are 30+ years old) with thickneses that vary from thick 1/4" to over 3/8", with gelcoat interior
. I've cleaned them over the years, but they need a good scrubbing to get the more dense, tenacious organic material removed. I've tried bleach, vinegar and water, but that doesn't get the dense material.
Maybe i can stiffen up the exterior where I cut the openings, then build a ... I don't know. I'll figure something out.