Plastic over fiberglass
Sounds like a heck of a lot of work, Sailorjack. I go with Peggie on this one.
The only application for which I'd consider using fiberglass is an integral tank, building it right into the inside of the boat. And this is best done at the building stage, as-- no matter what you use the tank for (and it shouldn't be for fresh water) --it will require some additional hull layup and a lot of work to fit and 'glass the top properly. We've done this with diesel tanks and holding tanks before. The main issue is making sure there is a throughway for bilge water to get past the tank.
Remember that modern ethanol-blend fuels and fiberglass tanks do NOT mix. If you use gasoline, plastic is your best (and only) option.
A friend has a 1960s boat with integral fresh-water tanks, and the keel bolts come up through the bottom of the fresh-water supply. It was designed this way. This was inexcusable. To inspect or maintain the keel bolts you have to drain the fresh-water supply, remove the lid covers (and their bedding compound), attend to the bolts and then wait for your 5200 to dry before refastening the lid and refilling the supply. Just imagine if the bolts were leaking seawater and you didn't know till you were at sea and having to drink or cook with that tank water. It's absurd-- yet many boats in the '60s were done this way.
As Peggie says, there are hundreds of weird and wild tank shapes to choose from. Ronco and Raritan both supply tanks in light- (fresh water) and heavy- (holding) duty plastic. These are engineer-designed and will pretty much last for ever. Few if any cost over $500. Given the option I wouldn't take out my toolbox for that.
Best of all, if it ever does go bad, since you know you got it down the hatch and into the bilge, you know you can get the old one out and get a replacement back in. Building custom tanks, especially when working inside the boat, can be like building a boat in your basement....
Just my two bits there :dance: