Rusty Fuel Tank - treatment

My 715B tank was cleaned 2 years back using vinegar and looked good but now problem back with rust clogging the small pickup port . 
  Solid but rust is a problem as can't keep tank full all the time and it seems the mix of bio-diesel added to fuel now promotes moisture and rust .
 Local place says they can coat tank but  when asked on price - quoted $450 and 2 year warranty . 
 Wondering on what coating holds up to diesel .  Few years back I bought a treatment for motorcycle tanks but have not used it . See several coating advertised and like most things they require a warm area to fully process and cure products . So might be a summer job when temp stays above 70F out 
 Will clean tank with Inhibited HCL acid to prep then and go from there . 
Might also add a larger outlet to tank and a pre filter screen in case of debris in tank .
  Odd have had machine now for 30 years and only last 2 years has this become a problem . Wish there was a way to just plate the inside of the tank with zinc or copper but guess a coating is the way - if it stands up to the fuel . 

I cleaned my tank by strapping it to a cement mixer, letting it turn over and over with stainless pins and stainless nuts of random sizes.
I removed a bunch of crud but I did not coat the inside.
After I reinstalled and was running, I stopped in Napa for a part and next to the counter was a cart with fuel tank coating.
Could have been a sign but I missed it.

Cleaning it is easy part , keeping it clean after is the hard thing 
Need something to line the bare metal to keep the rust from reforming .

Ran that link through WAYBACK MACHINE - Look up and it doesn't come up with article or site 

Something I have been thinking is making on myself in stainless or seeing if a poly tank of right size and shape can be found . 
 A real pain to try to clean inside of tank to only have same problem return over and over .
Reading some on coatings it seems flaking and adhesion problems occur as expansion / contraction from engine heat is a problem and as tank is mounted under hood and behind engine can be a problem with that happening . Not like a automotive or truck tank mounted out of engine compartment .

When I was hauling bean oil from processing plants in Iowa to refineries and tank farms for blending with diesel - it was said the bean oil was a great paint remover as well as other uses by itself . Unsure if it attracts and holds water but I know as sulphur content has been reduced in diesel and other GREEN IDEAS - the lubricity of what is sold as fuel has created problems , not only in on-road use but off-road as well . 
 Seems the diesel pumps used in automotive class pickups and lighter trucks has created a lot of problems and recalls .

the 715 fuel tank has bottom opening for suction and return , located as center of tank 
I was looking at it and sediment bowl is ahead of fuel pump but not no tank .
tapping a larger discharge line into tank - a valve - a filter - then to existing system might be the answer .

Odd how solving one so called problem creates 20 more . 
Alcohol was the big thing in additive to gasoline , then processing corn into Alcohol has now created a CO2 problem where it is going to be collected and piped to ND and released underground ?????
 Seems diesel had lubricity and not big problems with rust or absorbing water , now with low or Ultra Low Sulphur pump wear and other injector problems are common . 
 Same with the regen systems where DEF fluid is added - but at it's  - 20F here today the ICE in tank , pump , hoses for DEF system fail to work .