I just reinstalled a gas tank (1986 280GE) that I had a Renu franchise repair. I had no rust or scale, just a leak at the seam. Their process is a coating (exterior and interior) and bake on. For my application they had to cut a hole in the top of the tank to remove the small plastic tank at the inlet that helps to prevent air suction when fuel is at a low state (the name of it escapes me). They installed a metal one, welded the cut out piece back on, coated, baked and UPSed it back. Lifetime Warranty to original owner. After looking at it, I can't see how it could fail again.
POR-15 makes a reputable product that a number of folks I know have used and are satisfied with. Can't say that I've ever tried it. They have the chemicals to clean it and the actual coating would be your option. Good Luck and let us know what you do and how it turns out.
S, J.R. Brown
1986 280GE LWB Anthracite
1980 450SEL Champange
1981 Volvo 240DL UPS Brown