John,

Having replaced our tank, ( http://www.svguenevere.com/prep/proj/ftank/ ) I can tell you that you can NOT tell how good the tank is from an inspection of the inside surface!

If the tank can come out easy, inspect it and then, if you think it needs it, coat the outside with a type of glass that will flex. The tank does a lot of flexing as you fill, use, and sail.

Then put in another inspection port with the other pickups as needed.

Most likely about 2 or 3 boat bucks less expensive!

Greg