I'll tell you what I know from recent experience:
Bought a 35 year old boat always in humid Florida
, very neglected, 55 gallon mild steel diesel
tank, well supported with no damp contact points anywhere, 1/2 tank of fuel was watery and black, engine
not running for months.
Was of course, concerned as you, about the tank. Tank was too large to take out from engine
compartment and had no way to clean or inspect.
Decided to empty the watery black fuel, unstrap it, cut 2 large access holes on the top and deal with whatever I would find before cutting it in pieces and settle for 2 smaller new tanks and devise a way to support them.
Inside, there was only a 1/8 cup of real nasty, thicker soup made up of black algae and some fine sand at the bottom not able to be picked up by the suction tube.
The fuel screen
was still clear, meaning to say.... after 35 years of the worst conditions, if tank would have been just drained and refilled, it would still be an average tank, except if stirred up by lumpy seas, that 1/8 cup of crud would clog up a few filter changes.
What I did find was that the inside steel
had been painted ( not your case with a stainless tank ) by the manufacturer 35 years back, with a gray coating that by now, the lower 1/4 of the tank was soft and the gray would scrape off easily. Also there was about 6 square inches of surface, with small shallow pitting where most of the water would always puddle under the pick up tube.
Well, I scraped, sanded, degreased, etched and re-coated the interior with a special coating, put in a new fuel level sender, made 2 aluminum
bolt on inspection
covers with fuel proof gaskets and insulated the mounting bolts to avoid corrosion
, painted the outside and re-strapped the tank. As new now !.
Check the electrical
ground connection from the tank to the fuel fill inlet and change the 'o'ring at the fill often. Look closely where the tank rests on a support and see if it would ever be damp, watch for tiny cracks or the rusty color of crevice corrosion
. Same applies to an aluminum
tank, they also corrode when resting on a damp surface.
In your case, It might be good to first attach a rag onto a stiff wire and try to swab the bottom of the tank through the fuel fill if possible, after fully emptying the tank and look at the rag to judge the condition of the sludge.
friend found an old rag inside his fuel tank!.
Hope all is well with yours,as to avoid a very tedious job. Good luck.