The PSC 37 tank Bob mentioned is mine (1998) and it was luckily leaking when the boat was on the hard. However, it came as a surprise since when I purchased it 2 years ago the tank had already been leaking and repaired & I figured this would have been adequate.
Mainsail is right about the "production shavings" but in my case it was the saw dust and left over construction materials that were found under the tank in the bilge that kept it damp from below that contributed to the corrosion. I also wonder about boats laid up for the winter with water in the bilge some of which is salt water from the anchor. Sitting for months like this could also contribute to the corrosion. The tank sits on glassed in, half round, pvc pipe covering the keel bolts. You will have to remove the tank to find out about the corrosion since looking at it from above, it looks great.
The repair that was done did not include a primer of any kind and the corrosion was worse in this repair area and near the welds but then it is the lowest area of the tank and most subject to corrosion from the bilge water. Photos are attached.
A new tank from Pacific Seacraft is $1,950 plus shipping. The good news is that they are easy to remove and re-install. Cleaning up the diesel mess and pumping it out was much worse given my lack of experience in doing this. Don't bother trying a siphon.